When I converted mine I used a .15 OS 2 stroke. I had to cut off 1/4 of an inch off the fire wall to get it to fit in the cowl. As for power with a .15 on it at full power it flies to fast so I cruise mine around on half throttle. That gives me some extra power on the throttle if I get into trouble and need it. I don't think structurally the plane can handle a larger engine. On the .15 my champ lands about four clicks up from idle position.