Ah the FS1E, my first ever road bike.
A Micron "spanny", a 60cc "big bore" kit, a bit of porting & polishing, cut down rotary disc & different gearing resulted in the clock showing an indicated 71mph down the Mile before the engine went bang in quite a spectacular fashion
After that it received the engine from a Yamaha YB100 (straight fit and almost impossible to tell) courtesy of Chris from Speedway Motorcycles in Mitcham Lane.
Those were the days when it cost around £1.40 a gallon and plod didn't really care what you did as long as you weren't being a total arse when you were doing it.