I just picked up an ultra stick 40 today and am already about 3/4 the way done. Running a Saito 72. I chose not to use the the quad flaps as I would rarely use them. I have bigger tires, 3 1/4" that should help out grass landings and any prop clearance problems, I am going with a 13x4.

So the plane flys well? Is it stable at slower speeds or does it like to rock a lot.? Was the extra weight of the bigger engine worth it, however, I think my saito 72 is quite a bit ligher than your OS four storke.