Check for worn and or no armature brushes in the generator. If you pull them, or what's left of them, look closely at the commutator. The copper segments should be smooth and have no copper between segments. The commutator will likely need turning in a lathe anyway, then grind the kerf off a hacksaw blade and cut between the segments. Now, oin a lathe, smooth the commutator with fine grit emory cloth, pop in new brushes, and it shoukld be OK. Check for field coil continuity also.