I tried messing with a YS early on in my rc/pattern career. Great power until I started having problems, spent a lot of down time trying to solve, or waiting for engine to return from YS parts and service.

They are awesome engines if you have the time to learn and understand them. I learned the hard way that I'd rather have a reliable motor and spend my time concentrating on flying, not tuning.

OS 120AX is an awesome engine, Saito should be fine too. Personally I might try a pumped OS 4-stroke, but expensive and again asking for things to go wrong.

As you can see, I switched to electric power (still experimenting though)