My mate had the SS50, I went down the Fizzy route as they were a lot easier to tune.
After blowing it up twice going down the mile, I discovered that the YB100 engine slotted straight in to the Fizzy frame. In fact the engines were so similar, you wouldn't know unless you looked at the cubic capacity that was stamped on the barrel that it was no longer 49cc.
Come 17, I bought a KH250 (the S1 model) and what an animal that was (when it actually fired on all 3 pots that is). Thinking back, it doesn't surprise me that a lot of folk were hurting themselves...I'm sure it was a combination of having started on 2 wheels very early, combined with a bit of sheer luck and a LOT of grace from God that I am still here today to be honest.
Anyway, that was ditched 10 days later in favour of a GSX750 ET and I've just kinda gone on from there really.