It will fly just fine....Mine was built extremely light, and I ended up adding a ton of ballast in the nose with the smaller engine. It still flew fine. I was trying to think back on all the 1/4 scale cubs that I have flown, and I think only 1 had a 120 on it, and it was on floats... the others were anywhere from a Saito 91 to a G-38 ...........