To make a good fitting recommendation, you'll need to tell us what your expectations are for this plane. Do you just want to putt-putt around the sky and do the loopty-loop every so often, or do you want some semblance of vertical performance? If you choose the former, then any old weezy weedie would do just fine. With all that wing area, the Stinger will be a floater even if you build the short wing. OTOH, a 40-50cc should pull the Stinger straight up till it's outa sight - IF that's your sort of thing. The Stinger airframe is certainly strong enough to take up to a 50cc.

I disagree that the chainsaws are any more difficult to convert than the weedies. Even the Ryobi 31cc, which may seem like a straight strip down and bolt on, takes some cutting and trimming to get operational. Some of the chainsaws (Homie and Poulan, etc.) have bolt on parts readily available and require very little if any machining to finish. Up front cost, however, is usually a little higher.

My choice would be either a dual-ring Ryobi 31cc or a 33cc chainsaw, depending on what is available in your area. Don't worry about a little extra weight. Again, with the big ol' plank of a wing, you can throw on another 2lbs,and it won't even notice it.