The biggest problem is the Geranium transistors that are NOT IMPOSSIBLE, but difficult to find. I have powered up dozens of these little jems and found them all to still be working properly. The most common problem seems to be 9v battery snap replacement, and cleaning the volume pots.
I really don't think you need too much in the way of schematics, they were all pretty much a common 2,4,6,8,12 transistor design.
There was very little in them to go wrong with age, other than mechanical damage.
I guess that you could argue Electrolytics would need to be replaced...but again, every little radio that Ive put fresh batteries into has just powered up.