RE: Modify a 1/5 WACO for ryobi engine?
I think that you are going to be disappointed with how much engine is hanging out in the breeze with that much engine on the fifth scale WACO. I did my first one with the Maloney 1.25, and that was a tight fit. Not much power either. Mine now is getting the OS 1.60 twin (I know it is overkill, but it does have a throttle, and will all be inside the cowl.
Nothing looks worse (IMHO) than a pretty WACO with the engine hanging out of the cowl.