Go ahead and install larger diameter tires WITH the engine upgrade, this will help get the prop up a little higher and not cutting as much grass on your takeoff run as grass cutting slows down the engine RPM as well as the aircraft itself. Remember for each 1 inch of tire diameter increase you only gain 1/2 inch more prop clearence and if you go from a 10" prop to an 11' prop you just lost the grass clearence gain from the bigger tires. Your back in the grass cutting business! Do engine and tires but stay with the 10' prop as these planes have short landing gear (well you could build new longer mains and nose gear too and then use an 11' prop). It dosen't end does it ?!?!