Have a yerf dog 6.5 hp I purchased used for the kids many years ago (now 26 and 24), and getting it back up and about for future grandkids(?). I live here in Southern Illinois, perfect go-carting weather and terrain.
They "grew out" of the kart and stopped? This is the wrong answer--you never outgrow go karts. They were invented by grown up men to race, or just to go fast in a small footprint.