It's been a little while since I tried this, but with battery power to the armature ground the field and it should start to turn or motor.
Now, unhook the field wire and it will speed up.
If it will do this the generator is good. I think it helps to open it up and polish commutator and also sand and clean the iron inside on the field coils and rotating iron. I'm not sure about paint but at least oil it some. I think the rust inhibits them some.
JohnT on the tractor board has a trouble shooting guide but I don't know the link right now.
RT (I hope I don't need this but don't oil the commutator.)