unless your a big boy (like me) 3-5hp has been powering go carts for as long as I remember. I think the bigger motors really help more with not bogging when mowing grass, and full out racing of course.
with some 4hp weighing in at 32lbs, 3.5hp at 26lbs with a light rider would probably be pretty fun, especially good handling if it found itself on a light well setup mower too.... learn how to weld aluminum, and make the AlumiMower!