My Shoestring has a OS 91fs and has plenty of power. I would think a OS .61fx would have enough power also. This is not a heavy airplane. Reinforce The landing gear mount. Have Cub yellow monocote on hand. This plane carries it's speed well but will slow down. Just takes longer. When reading the Shoestring flight instructions it tells you the plane has a tendency to bounce on landing. It does, so just be prepared to nail the throttle and go around. Better than stalling it in after a big bounce. The Shoestring is one of the best ARFs around. You WILL love it!!!