If it's an old OMC saildrive you're having trouble with, they are notorious for being troublesome, and there are many many boats sailing around with outboards on the transom that originally came with these things installed.
I suspect that a good many of them have outboards on the transom and continue to drag the original saildrive leg around rather than go to the bother of removal and reglassing.
While replacing with another diesel powered saildrive setup would be ideal, even then there'd be some hull work required and the cost would spiral upward in a hurry.. possibly to beyond what the boat's worth... doubtless why most go the outboard route.
If you do go the outboard route, do it right; get a long shaft, a good mount that is easy to use/access, be sure you can run the gear and throttle conveniently from the cockpit. Also check into whether you need to reinforce the transom area where you plan to mount the bracket.. lots of intertia here with a heavyish motor slamming up and down in a seaway. And, as SD suggests, ideally remove and reglass the original installation to get rid of weight and drag.