You ask "he installed a 12 volt battery, negative ground, but did not make any other changes. What may I expect to be
damaged because of this?
If all he did was install a 12 volt battery into a 6 volt tractor (if that's what it was??) but it never started:
1) If 12 volts was applied to an old 6 volt voltage regulator (if that's what it had???) for a long enough time period it could possibly be
damaged.
2) If polarity were reversed (Pos to Neg ground if that's what it was???) and the generator never got correctly polarized, again that can
damage a voltage regulator
3) 12 volts on an old 6 volt coil (IF thats what it had and assuming no ballast was added) can overheat and damage it given enough time, but
if it never started it may well be okay.
4) 12 volts on a 6 volt coil can eventually cause premature points burning if the ignition was left on long enough. (true for even correct
voltage ign on long time not running)
5) Of course if 12 volts was applied to any 6 volt lights (if it had such) they will glow very bright a short time then burn out
If its a 6 volt tractor and has a 6 volt coil you can easily add an external series voltage dropping (12 to 6) ballast resistor (or else toss the old coil and get a new full true 12 volt coil),,,,,,,, get a 12 volt voltage regulator to match the tractors ground,,,,,,,,,,get a 12 volt generator,,,,,,,,,polarize the generator to match the ground,,,,,,,,,,,,,,get 12 volt lights,,,,,,,,,if it has an ammeter and polarity got swapped its leads need reversed,,,,,,,,,,,,,wire coil to match tractors polarity. NOTE I have used 6 volt gennys on a 12 volt system and only changed the VR and it worked
John T