I have a weed trimmer I won't be using anymore, it's almost brand new, the engine is a Ryobi 26.2 cc 4-stroke. Is this convertable? I'm afraid it might not have enough power to do much of anything compared to a typical 20-25cc 2-stroke conversion.

I have one on the shelf to try out.
It is a bit overweight, and low in power, but would do nicely on a large cub while sitting in a lazy chair with the transmitter by my side on the ground.
Some day I will find the time for this project.

Check my post in Tach Readings and Engine Conversions on this engine. Might just surprise you!