Do you know what happened to the generator? Often it's just brushes, and you can replace these. Or an internal connection, which can be repaired. If a field coil is open- or short-circuited, something is dramatically fried, or an armature winding is shorted to the body, you can't do much, but these are not so common.
Also, be sure it's really the generator that's bad, not the regulator.