Boy, you *are* opening up a can of worms!
(and I've had that can thrown at me many times /bcforum/images/%%GRAEMLIN_URL%%/jester.gif ).
One of things that will give you information is to use the "Search" function here at BCF. That will provide a lot of info that is already out there.
Anyway, for a street car, the lever units are fine. When I hunted down a Midget for my own brother a few years ago, he asked about convering to tube and I told him that for a street toy, the lever dampers are fine.
A nice thing to do with them is to flush them out *fully* (not just top them up) and add fresh shock oil.
And the question of what shock oil to use in the lever units is it's own can of worms...to be safe, I'd use the stuff from Moss or Vic/Brit.....or use motorcycle fork oil
And if you have leaky or worn dampers, Peter C (a member here) has an excellent reputation for rebuilding these units. Otherwise all I can say is that the other popular rebuilder (located near me) has a poor reputation for quality.
Note how I am not "naming names" here....dumping on vendors by name is considered "bad form" here at BCF
If you are racing or doing heavy autocross, the tube shocks are the choice. I cannot think of a non-vintage racer Spridget that is running lever dampers anymore.
If you would like to see the method that I used for fitting the front levers, you can
~Click Here~
By the way, the rear dampers can be switched to tube-type by flipping the lower leaf spring plate and fitting a properly positioned mounting bolt to the chassis. I replaced the axle strap mounting bolt with an upper shock mount (and moved the upper axle strap bolt).
You can see my left rear shock here (that's the battery in front of it...the arrow is just pointing to the panhard rod end)
[Also~The reason you have to scroll over to view some of these posts is because the camera phone picture is too large and is messing up the browser window]