From what I see out there most servo repairs / rebuilds involve replacing the gears and sometimes the case.

There are some other parts though, some models have replacement mechanics which seems to be just the motor assembly. Not seen any replacement circuit boards, but then again have not been looking.

If it were me and servo seemed to have an electronic failure I would shop for a new servo, I have not been the type to buy the most expensive servos in the first place and as long as I run a metal gear model I have pretty good luck.