Oooh cracked receiver wouldn't be good. From everything I've heard, the post-war models' quality was not on par with the wartime guns. If you get your hands on these guns and it looks like it may not even be worth spending money on a 'smith, I'd gladly take a look at them for you. I don't call myself a "guru" or anything of the sort, but I've fixed plenty of my own guns including my '45 Underwood M1 Carbine.
Most common problems with the carbine:
Feeding: Crappy aftermarket magazines of which there are plenty of out there.
Cycling: Loose or worn gas piston/nut.
Ejecting: Gunked-up extractor spring channel, extractor plunger not seated correctly at it's divot in the extractor, weak extractor spring.
I have the carbine bolt tool you can borrow if you like.