I flew my original JR Explorer ( AKA Excelleron) with a used YS91AC and it flew fine. Could easily do the sportsman sequence. I think it would be short on the vertical maneuvers in Intermediate. Naturally I later stepped up to the YS110 and loved the power. If you have the 91 already, give it a try. At least if you decide to step up to the 110 it's just a bolt in replacement.
Eddie