Modifying an engine can be done but it usually hurts things like idle and longevity. Most .75s are based on a .60 equal, so a change would be easy. A .40size to a .60 power would be more what you are talking about because of the case size differences. Unless you fly pylon modifying engines usually hurts more than helps. Not much has been done with 4 strokes though mainly because cams take special equipment to reproduce.