its probably a ryobi 31cc,the size and rear carb say ryobi.is the carb mounted in a plastic back plate? its a ryobi. check out wacker engines .com for parts such as prop hub mount muffler etc.its a fair engine to convert power wise.
recomended is 1000 sqr inches of wing no more than 14 lbs.im still working on mine ,building the plane.
good luck