I“ve used Merco 35, 49-50 and 61 in competition during several years with complete satisfaction, Merco 35 is approx 1,5 oz heavier than a Fox, too, but they are mild, easy to use and with a flat torque and lovable break.
Just adjust the nose lenght while biulding your stunter!!!
Tom Dixon has huge experience with them in both Classic and F2B flying.
I like these engines.
Regards.