I dont want to go too far out on this particular limb, because what may be o.k. for one
guy may be no where near enough for the next .I am presently building a 31cc ryobi for
a 72"yak54 at 12 lbs. and Im being told that Im underpowered .
There are way too many naysayers.
A 25cc engine will fly a 12-pound airplane with authority. I know for sure that a 25cc engine will fly a 25-pound airplane however somewhat dangerously. Much of the way an airplane flies depends on your ability to prop the engine and your ability to tune or modify the engine.
The entire process of converting engines can be viewed as a giant ongoing design experiment. Have fun with the conversion. If after flying you don’t have enough power convert another bigger engine. You will have fun converting the second engine.
Bill