O.S. 20 four strokes sound great! Good choice for the LW Champ too. Mine flew so well with an O.S. 15 FP I'm sure your 20 FS work be fine too. You can use more power too, of course. I'm not surprised a Fox 36 was good, but it also flies very well with a lot less power. You just have to watch the weight if you use a small motor. Mine was not quite 3 lbs.
Jim