For a half throttle hover a 4 stroke 120 wont be enough. My ultimate hovered with an OS 120fs at nearly full throttle, sure it was do-able but the safety just wasn't there. The airframe is perfectly strong enough stock for the 120 and a YS 120 was what the designer Dave Patrick used in his "wring it out" series of tapes. The only mod I would do is put in the two wing servos. Currently I fly my stock ultimate with a saito 150 which I feel is close to the perfect match for this plane, it hovers just about half but the pull-out is much better and its general flight characteristics are excellent. My votes for the saito 150, my airframes had about 8 years of flight with big four strokes and is still holding together.