Here's some photos of a Ryobi I found. I think av8tor was saying this Ryobi isn't the most powerful of the 31 cc engines, maybe someone can say if it is or isn't (because of the bar in the exhaust?). The trimmer that it's on runs poorly and smokes a lot. Do you think the rings are bad? Here's some photos of the cylinder wall too.
The Homelite 25cc engines I have might be better for this size plane. One of those starts fine, acts like it needs a carb rebuild, and the other one smokes like a train. I'll try to get some photos of the one I choose this weekend.