RE: .20 size rear exhaust hi-po engine?
Check Nova Rossi. They have several 21 marine motors that you could order an aero head button and finned head for. Hang on though cause you are talking about 2-3hp and over 34000rpms.
They also make an aero side port motor that is right at 2hp and 30000 rpms. I have one I converted to marine and will tell you it is a real strong motor. I believe it will serve your purpose better and there is no need to convert it. Has a nice muffler and sells for about $125 I think. Look at the NOva Rossi site for it.
Regards Curt