RE: 4 stroke
4 strokes are difficult to watercool (read it from diff. posts). actualy if it is for a playboat, aircooled will be ok as long as you design the hatch so that air can freely enter and exit, that's what i've been told when i ask if its ok to run it aircooled inside a hatch. (remember i have not build yet). i agree with it though, you dont really need air rushing towards the engine since the fins on the flywheel are the once supplying the cooling air to the engine.