If you mean the small gear in the end of the starter, then yes you can have the drive replaced in the starter motor and not replace the whole assembly. As for the ring gear I can not remember exactly, so if any body else knows better feel free to correct me. Usually when a ring gear is new the side of the gear that the starter engages on will have the leading edge tapered back slightly so that it engages smooth, so I would think that you would be better to replace it. If they are not too bad I have gone aroung with a die grinder and small carbide bit and cleaned the teeth up but maybe it is worse than that.
Goalie, I think, if you can buy the gear, you will have to apply heat to remove it and then reverse the process to reinstall it. This will be like changing tires on an old steam train engine's drive wheels. It might be hard to find one of those old boys to help you out. I like what an old man told me about an angry engine I once had in for repair. He said, "If man made it I can fix it. If God made it sometimes I can and sometimes I can't." He fixed my engine and he was 92 years old. It shouldn't cost anymore than $80-$100 to repair the motor and put a bendix gear on it. The ring gear I don't have an idea but I think the post on the leading edge being there is correct. Good luck. Going to go watch the sugar cane burn today and enjoy the 80F temps. That will fix most things.