It is possible that the slip rings (where the brushes make contact) are worn.

Replacing the rings is possible but requires some skill and tools.

If the alternator was not replaced/rebuild in recent years I would replace it with a rebuild one. Don't buy a used one, probably just as old and worn.

It is easy to check the rings, just remove the regulator with the brushes and feel the rings with you finger
On this page you can see the difference between the worn and new rings. Not the same alternator but the principle is the same.

Second I would not trust a used regulator, the brushes may be ok but one of the rectifiers may be blown.

ROb