Many years ago, I had a Fox Hawk .60 engine on one of those Dumas Airboats like yours. I had to be careful as it had more than enough power to go airborne, but it didn't have a elevator or ailerons on it to control it with when airborne of course (also it would be quite tail heavy too). I remember the kit saying for up to .60 engines, so that was what I used. It went plenty fast at half throttle.