I have a US 41 engine in my Decathlon. It is rated at 3 hp and weighs about 4 and 1/2 pounds. It made the plane balance perfect and the spring starter spaces it forward just the right amount. I mix my fuel 32:1 and I get NO fuel residue anywhere on my plane. My Decath. weighs 18 pounds ready to fly. I build a little on the heavy side and it also has a scaled out interior and a full size pilot. I would not consider putting any smaller engine in my Decath, unless I wanted to just fly in large slow circles. The US 41 seems to be a perfect match for it. It uses a Top Flite 18X6-10 prop which gives plenty of ground clearance.

Bob