I had a craftsman one, I think it was 14hp ish. I got for $40, I found an old wrecked dirt bike, and stole its bars, and used the twist grip throttle and clutch and brake. Pulled the governor, changed up the pulleys for some extra top end, and did some mild exhaust work. Welded the front beam solid. and drilled a hole through the rear end put a bolt in it as a "locker"
it would go about 35-40mph flat out, and would get the front wheels off the ground about 4" if you popped the clutch just right, wouldn't carry it at all though. Faster then the 70cc 4 stroke pit bike my neighbor had at the time.
If you search there is a whole racing lawnmower league and shit. Never got involved with that, but I did see them race at a local fair, it gave me the inspiration for the dirtbike style steering and controls.
All in all it was a great $100.