I didn't build it but I flew one last weekend that my flyng buddy built. He used the K&B .61 (new style twist on cylinder w/stock pipe/muffler) and built as per instructions. It needed just over 3 oz. weight on the nose to balance at the rear of the recommended CG. It tracked well on the water and got on the step very easily and flew great. Then he said for me to wring it out which to my thinking is "what can a 7+ lb. seaplane do?" I couldn't do upright flat spins very well but inverted it would hold altitude, hover, torque rolls would fall out to the positive side. Knife edge flight was phenomenal-------slight elevator inputs needed but it was like a fun-fly plane, inside and outside turns and would gain altitude with just a bit of throttle and rudder input. Did manage a knife edge loop but it wasn't very pretty. I am presently building a Northstar which I know from others I've seen and flew will not perform like my friends Sea Monster. I will have one of these this Spring and will feel secure in using my old OS-.61 SF long stroke on it.
Go for it with this plane. Just don't let the secret out as to how well they fly because I want the availability to be there when I order mine.