Before doing anything to a 30 year old fiberglass/wood boat, check the structure for rot!
Also, as Scott said, OMC has been out of business about 20 years, yes you can still get most Cobra parts but it is very hard to find techs who can or will work on them. I do all my own maintenance, there are 2 shops around here who will work on them. If the boat is solid, you could just repower it with a GM marine 3.0. If you want to go bigger, the biggest I'd go is a 4.3 V6 which is shorter but wider than the 3.0. At the same time you'd have to fabricate new mounts for the V6 engine and while you're at it, I'd sell off all the OMC stuff on ebay and just buy a pulleys to prop package Merc or Volvo package. In the long run, best bet if you must keep the boat.