I ran an OS 91 on mine. Flew it fine with 16x4W APC, but no hovering maneuvers. Another guy at the field has one with a pumped OS 91 and 14x6MA. Again flies fine, but won't hover. I agree with all who have suggested the YS--great power. The Saito 125 should be OK too. No worries with inverted engines--just have to adjust low end.

Greg