Is it the 20 cu twin or a 3 cyl.? You can always polish out the ports a little and relieve back pressure by drilling a few larger holes in the exhaust housing, but that engine has been everything from an 18 to a 35 HP and it is near the limit because of displacement.
I am guessing it is a 20 cu in twin. I defiantly know it is not a 3 cyl. I havent ever heard of anybody doing anything to really hype up smaller motors and figured I would ask. It would defiantly be cool to pick up some hp's though. I need to find out what the right prop is for the boat. I got one that seems to do fine until the boat is slam loaded and got one that does great loaded, but runs to high of rpms when its light.