:) Ah well, sounds like you are about to start a challenge then.
From memory, OS four strokes have an awkward king of shape to cylinder, (for wrapping copper pipe round!), they are "squared off", and the push rod tubes run in front of the cylinder, so might be good to avoid.
You are right about the noise though, should be reallly quiet in a boat.
Prop size will need thought, as the engine power delivery characteristics will be different to a more conventional two stroke motor.
I (and I am sure lots of others here) will be very interested to see photos of the completed conversion, and to see an update on operational progress.
Good luck and have fun.
Glenn