Slick craft boats were well made, what you have there is a 68-72 model. I had a customer with that same boat when I first got into the business. If the hull is still sound pull everything out and have the transom closed, locate a Mercruiser late style drive platform so that you can use the engine over.
Or get a bracket and bolt on an outboard with about 200 hp which might be the easiest and cheapest way to go in the long run.