I think my first job would be to strip it down and see what the problem is, it might just be a broken spring or suchlike, in which case an easy fix. These drives were used a lot on narrow boats which is where I first encountered them, Chertsey Meads Marine may be a good source of advice and/or parts. If you are stuck, I'm sure there are club members who are adept and would lend a hand.
Terry.