If it's been rewound for 12v then use a 12 volt reg, if you aren't sure about the amps pick a lower one just to be safe. The critical deal with generators is what the regulator is trying to make the generator do, if it's rated for 30A and you use a 38A bus reg it'll punish it until the solder melts out of the commutator by constantly demanding more amps. The regulator can't be rated higher than what the generator can do.