I bought my boy a new Kawasaki 50 4-wheeler when he was a kid, it had restrictor in the exhaust in the head, and a throttle limiter.
Took care of that real quick it was a dog when new, after removing the plate, jetting the carb, milling .040" off the head and added a FMF full exhaust, it would ride wheelies with me on it.
Wife thought I was nuts when I tore the engine apart on a 2 month old kids toy.