Since I am NOT a purist, and I do drive my Sprites, I have been constantly trying to improve on the basic mechanicals.
What have others done with alternate parts to improve their Spridget?

I am running a Geo Metro aluminum radiator, just about a drop in and even uses the stock rad hoses.
Very small Subaru 1 wire alternator. all my LBCs are running some sort of modern alternator.
I have tried the Nissan Sentra Hitachi dizzy and it works fine but I also managed to adapt an Isuzu Trooper dizzy and I find it much smoother, maybe because it has a ball bearinf advance plate.
Datsun 5 speed, my A40 has a Toyota 5 speed coupled to a 1275.
I have the delayed wiper set up which is great.
And I found VW seat belts work just fine for 3 point belts.
Weber carb, transistorized electric fuel pump (not the noisy square job) and probably a few other goodies I can't think of right now.
I am looking into a modern brake master set up and a modern heater blower unit.
Oh yeah, you can put air conditioning in a Spridget if you really want to. I had it in my A40, straight out of an Austin Marina but adapted to the A series engine in about an afternoon. So it will work in a spridget.

What have others done to modernize their Spridgets?

I started with a '78 1500 RB car. I have chrome bumpers that I occasionally put on, but usually, it has none. Engine is stock internally and uses normal 4-speed all-synchro trans and open 3.91 final drive. Car is street driven (has full street equipment and "Historic" tags), but is primarily used for club racing and time trials. It has a full cage, fuel-cell and other race-stuff. I am using it in a class that doesn't allow "wild" changes, so it's pretty docile and works fine on unleaded 93 octane. It still has a fold-up top too.

Changes/Additions:

Mitsubishi alternator with large diameter (slow-turn) pully.
Electronic ignition (Pertronix) in 25D distributor with vacuum advance removed.
Solid core copper ignition wires (a radio was never even a consideration).
Mitsubishi electric fan with manual switch (I machined the fan-clutch hub off the water pump).
Monroe gas/oil tube shocks all around (front lever shocks in place but without valves and slotted to allow 3 degrees neg camber).
Summit racing seat (the only seat in the car)
Fender-mounted kill switch.
Re-cored radiator with extra tubes.
G-Force harness (5 point + arm restraints).
Mostly American toggle switches on dash.
A tee-fitting to allow an oil pressure gauge and light (the gauge is from a Piper Cub).
The noisy, square fuel pump + pressure regulator.
Kumho V700 DOT-legal shaved radials (on normal Rostyles).
Throttle cable stolen from one of the kid's old 10-speed bikes.
3/4" front sway bar.
5/8" rear sway bar + custom panhard rod.
8-leaf half-elliptics
A grill made from cool stainless steel mesh I found at the junkyard.
Cherry-bomb muffler attached to left-side sill (at the jack point). Easily removed with pins.
S&S 2-1/16" Harley Davidson "pumper" carb on hogged-out, re-welded single carb manifold (our club requires me to run a single carb on the 1500, so this is the biggest side-draft I could easily find.....1275s may run two H4s).
PaceSetter header (a very low quality item, in my view).
Minifin aluminum rear drums.
Fiamm air horns (just for fun).
Homebuilt, spin-on oil filter adapter.
Shortend steering column with quick-release 10" wheel (the key switch / lock is long-gone).

I'm planning on adding a Wildwood master cylinder (from Summit at about $50) plus MGB front calipers and Spitfire rotors (with a custom spacer)....most of my fellow Spridget racers already have this.
I'd like to add a rear-disk brake setup at some point (I'm not concerned about a handbrake).
Car still has the padded dash and tiny, little tach...this stuff will go.
One of the fellows has a Sprite with an alternator running of the rear of the trans....I might do this if time allows.

I switched over to neg. ground in my 59 bugeye a few years ago, and at that time I installed an alternator from a Nissan P/U truck and this has been totally trouble-free since installation! I also took the radiator out of my 77 VW Scirocco that I was junking and installed it. This required cutting the stock Sprite radiator mounts
and fabricating a new set-up to hold it in place. The car has never overheated since, and this is in S. Fla., even sitting in traffic in mid-summer!
I rewired the whole car, installing a Fiat fuse block w/10 fuses so that now everything is protected independently. If I blow a fuse,, the whole car doesn't die. Oh yes, I also found that a Harley Davidson muffler (its about the same dimensions as a glass pack) works really nicely providing a nice exhaust note with a nice flow-through design. I have a set of older (I'm not sure what year they came from)Corvette seats installed which fit very nicely and provide the same kind of seat-of-the-pants feel as the originals and are actually provide a slightly lower seating position. Of course, all the original parts are still in the garage if I ever want to go back, but the reliability for everyday driving is much improved.

Over the years I have heard of a few and actually seen a couple of Sprites with 4 wheel disc brakes (years ago at the Glen on a factory Sebring Sprite). Does anyone out there know what parts to use for converting the rear to discs?

Given that spridgets are so light, would rear discs make that much difference? I can see the need if you have overly large/wide tires, but on a fast road machine (with close to original tires)I would think that drums would be sufficient. I'm always interested in performance modifications, but never had given rear discs much thought. Can much stopping distance improv. be gained by using rear discs?

On a road car, you don't need to mess with the rear brakes - unless you're gonna run a proportioning valve that you can control from the driver's seat!

For my 'road' car, '59 Bugeye, I am not a purist. I have a 40 DCOE Weber, 3-1 header & glasspack, converted to neg ground, Petronix in a 23D dizzy, Motolita steering wheel, Roll bar, Lotus like racing stripes, electric (German) fuel pump, 5 point harness (drivers seat) regular seat belt (VW) for the passenger, Driving lights, Wind blocker, forward tilt bonnet, custom center counsel and center arm rest (both with internal storage). All these mods are bolt ons with no permenant modifications other than a few extra screw/bolt holes (the purist in me enters here) so that it could be reverted to stock configuration very easily. The only mod to the dash was the fitting of a 'Lucas Smoke Gauge' to fill a PO Ampmeter hole. Future plans call for replacing the 948 with a mild 1275 and 5 speed conversion along with a 3.9 rear.

OOPS, I forgot to add a front sway bar, negative offset king pins, relays and seperate fuses for the headlights(high and low beams), driving lights, and air horns, I also converted to a spin on oil filter, and run 5.5" mag wheels with fake 3 point knockoffs(couldn't find center caps).

Every time I log on and read some of all your additions, I recall more of mine, i.e. 1275, rib case, spin-on oil filter, disc brake front end, Sunbeam Tiger flip-up gas filler.

There are several conversions available to put rear disc brakes on a Sprite. Winners Circle sells a particularly good one.
But, as mentioned, they are totally unnecessary on a street car, and, I'm still running drums on the rear on the race car. I race primarily at a 1.5 mile, 13 turn course, that is extremely tight, and I use the brakes hard, and have never had a problem with them.
But if you really want to go overboard, you run discs on the rear, (with an adjustable proportioning valve) and MGB calipers on the front, with Spitfire rotors. Just make sure your belts are tight, or you'll be French kissing the windshield the first time you make a panic stop!
Jeff

I will heed good advice. I was just wondering on the merits of rear wheel discs, having seen a couple in the past. Since I put the disc front end on I must say that the braking is more than adequate. I don't love the windscreen so much as to want to kiss it!