A Webra 50 will handle it and is probably the lightest that will. Add an ultrathrust muffler to it and it'll really handle it. Do the same thing to the OS50 and it'll come alive too. I don't know if there is an ultrathrust for Irvine 53.

My Pizazz is 5.5 lbs and I've flown it with both the Saito 72 and YS63 on 15% and 30% nitro. It will hover with either engine on 15% and have just enough power to jerk it straight when things get a little crooked or pull it straight up out of the hover. But just enough. 30% nitro helps a lot but I would say (IMHO) that 5.5 lbs is the upper limit for both of these engines and still have good authority in the hover.