I had a proper go on my Pit Bike at Cheddar on Sunday (after my previous effort ended with failed brakes and a big accident).
The bike was *way* too slow for me. I spoke to the chap who runs Miniature Parts and he reckoned 125s are just slow. I was originally going to buy a YX140 from him, but I found this 2-year-old "barely-used" 155z on eBay and managed to pick it up for £180.
It sounds like a beast!
https://www.youtube.com/watch?v=w2RMs1oYcCw
EDIT: I now have a perfectly good YX125 (?) engine available for sale. It's had about an hour's use, if that, since I put a new piston and rings in it. Hit me up if you're interested...