I agree with the idea of fixing the engine. You will be far better off going with the original power source. Remember, you are dealing with a 10,000 pound boat dry weight. It is probably over 12,000 in cruising weight with full equipment, water, and supplies.
One thing that the Catalina 30 tends to do in waves is it can hobby horse. This will make the outboard far more inefficient and it will bring the prop out of the water, reving the engine to high RPMs.
The engines on these boats aren't too bad to work on. There is a ton of information out there on them. Do yourself a favor and stick with what works and how the boat was designed.