Almost anything can be rebuilt, especially when it comes to TRactor motors. I once had one that had been so badly neglected that the bearings were worn into the copper backing, and it ran great after having the crank turned and replacing bearings (including cam bearings), pistons & pins, liners, timing chain & gears, camshaft, valves, valve guides, valve springs, & the rocker shaft. In fact, I wish had that motor back, as the "3/4 race" cam & cheap header from JC Whitney really woke it up.
For the bottom end, the most important consideration is the crankshaft surfaces, so start there. Any scoring or wear big enough to feel with a fingernail means it needs to be turned, so you can stop right there and shlep it off to the nearest machine shop for attention. I'd suggest National Crankshaft in Anaheim, but I understand they have changed hands and no longer take retail business directly. Clark's Discount auto parts in Bellflower would send it out for a good price, but I don't know where they send them. Whoever you go to, ask them to Magnaflux it first.
Plastigage only reports what the actual gap is; it won't tell you if the gap is the result of worn bearings or worn journals. So it's really only useful as a final check before assembling with new bearings. You'll need calipers or (much better) a micrometer to actually check the journal diameters; but using a micrometer is something of an art in itself. Don't know if it's still true, but it used to be that a "full service" auto parts stores (like NAPA) would measure it for you.
Otherwise, at least for an initial inspection, I would just look for any obvious signs of damage or excessive wear. If it has spun a bearing, or a thrust washer fallen out, the damage will be obvious. Check out the bottoms of the lifters, any visible wear or damage here will probably mean replacing (or regrinding) the cam & lifters. If the cylinder bores look nearly perfect, they can probably be reused. To get a better idea of bore condition, remove a piston ring from one of the pistons, and use the piston to push the ring to the bottom of the bore. Measure the end gap to get an idea of the ring condition; then repeat the process at the middle and the top of the bore to judge the bore condition. The worst bore wear is always just below the top, where the top ring is when the cylinder fires; there will be almost no wear whatsoever at the very bottom (where the rings don't run at all).
There's more, but I'll let others fill that in.
Oh yeah, having a copy of the factory workshop manual (or Bentley reprint) is a must, IMO. Let me know if you don't have one and we can work something out. The Haynes manual is a close second best, IMO, but watch out for misprints.