RE: Ultra Stick 120[not Lite]and Ryobi 31cc
I have a 120 size stick, home-built from ply that weighs 17 pounds with the Mac 32cc installed, swinging a 18x10 prop. It has plenty of power, and runs great.....This plane is probably a little heavier than the regular "stick" so you should be fine with an 18x10......See pictures under "frankenstein" link (conversions).