i found a place that had like rtary engines that could get about 12 horse from a cingle rotor in the size of a 3.5 hp piston engine without thwe piston sticking out. it as like 2000$ american tho.
12HP is not very much. Back in the day, Sachs made a bunch of single rotor "lawnmower" style engines for snowmobiles, etc. They made about 22HP from an engine the size of a case of beer. They come up on eBay all the time, and I keep meaning to buy one for a go-kart.