Hi Walter, your engine is indeed a 2GM20F. Max RPM is 3600 but only for short bursts, your cruising sweetspot will be 75 to 80% of your max RPM, so figure around 2700 to 2850 RPM.
I'll start by saying hope you get this. You are right on with the sweet spot. I now have a Kiwi Feathering prop. It has lowered my sweet spot. Are you the wikipedia of C&C 30. Ha Ha Thanks