Most watermakers are pretty much the same: filters, lift pump, pressure pump, membrane, control valve. I would ask around your local marina or boatyard for someone with a good general reputation in that field and see what he has to say. Alternatively post a description with some pics and someone here can probably talk you through it. I would be interested to see what an early 80' watermaker looked like.
If all else fails, read the instructions
If it ain't broke, don't fix it