It would be more fun for a weekend warrior to take up the challenge with input from Carlo and/or myself with others welcome to contribute as well.
I don't want to come across as a killjoy or negative in encouraging such project but, If we look back say ... 15 years or more removing the compact case Zemoah PU series engines from this equation leaving NOTHING but weedeater/blower/saw motors, we can better focus on what YOU the budget r/c boater has to work with. Newer offerings in current engines will be equal too or alike enough be in the mix as well.
Intended application of original engines one might choose plays a HUGE part so durability, RPM limits electrically governed or limited because of little to no meat cast into cylinder limiting breathing, or just cast so thin there is nothing than can be removed attempting porting work.
When we find an engine that allows some tinkering we then have our guinea pig
Being in the field commercially working on Chainsaws, Lawn Garden 2 strokes for over 30 years, tuning and tweaking looking for ways in extracting more power from per-say "It's all I had for an engine" ... have been there and done that.
My ability and willingness to mentor exceeds that of actually getting hands on involved doing such work any longer. So if anyone wants to dive into the fox hole and see just how deep it go's, We at MGB and our contributors will be more than willing to help you educate yourself further.
Regards,
MGB Team ....