There's been several guys on here that have stated that their U-Can-Do 3D flies great with a Saito 100. I'm sure the YS 91 will have as much juice as the Saito 100, probably a little more. I have this plane with a Saito 100, haven't flown it yet. I went with the Saito instead of the YS for the weight savings. If the engines were close in weight I would have bought the YS. I was just trying to be as light as possible. But, if I'd had a YS 91 on hand I would have used it without blinking. I think either of these engines will fly the plane well. The YS weighs 6 or so more ounces than the Saito 100, but after what I've read on here I don't think it will be a problem at all. I've seen it fly with an OS 91FX and it flew real nice with plenty of power. I'd say a ST G90 would fly it well. I think a 1.20 in either 2 or 4 stroke would be a bit much, but if you want a rocket go for it. I saw a guy fly one with a Saito 1.80, there was a lot of vibration making it's way to the control surfaces with that monster on it! Talk about overkill! I'd read up on the weak landing gear block. This area needs a little reinforcement. There's a couple of threads here on RCU that goes into detail about this and a few other things. You may have already read them. This is just my opinion, hope it helps in some way. Gosh, I feel like I'm writing a book here!