Now here's a question of engine compatibility that I don't believe has been discussed. Correct me if I'm wrong, though.
GM makes (or, has produced at one time) a marine application 4-cylinder in a 3.0L output, rated at 140 horsepower. Their 3.0L Mercruiser.
My father is retired, with waaaaaaay too much time on his hands, and wondered about the possibilities and problems that could be encountered with such a swap. I'm thinking transmission bellhousing, etc. Yet, they are both GM products, so it might not be as difficult as it sounds. Give me a reason here to point out the obvious to him, and sink this notion quicker than a leaky cement barge, please.