the plane is designed around a saito. If you are using the OS ou may have to redrill the holes and may have to drop it down a bit so you don't have to cut your cowel all to pieces. Busted's pic showes just how mine is cut up and notice how close to the top of the cowel it sets? I think an OS has a bigger head. I would not go no smaller than a 120 OS or 125 Saito. Also check very carefully the blind nuts inside the fusealage that hold the cabains on. On mine 2 of them were not threaded. and guess which ones they were ! The ones closest to the firewall. and boy what a booger they were to replace !!!! I have 5 flights on mine and really like it. I am still messing with the cg so nothing to report on that but if you go with a 2700 mah 6 volt pack dont place it in the nose section. especially if you use the OS.