Initial research indicates it is a a 1961 T20SL (Scrambler with lights / referred to in some places as a Sports Cub) engine in a 1955 frame. The frame is obviously wrong for the motor, but it seems to have the correct quickly detachable headlight, heavy duty forks and a few other little bits that go with the SL style. It is not entirely mine, no, this is a joint ownership arrangement, like people do with highly strung and very valuable race horses.
The SL engine does indeed have a bit of a racy background, with a compression ratio of 9:1 (the standard T20 road Cub is 7:1), the 'R' cam, sports type valve springs, close ratio gears and flat styleee handlebars. Just imagine crouching down over the tank, elbows tucked in, hanging on to those bars for all you're worth and hitting the top speed of 77.8mph...as reported by 'Motor Cycling' in a road test in 1961. That's another 13mph faster than the T20.
All I need to do now is make a bit of space in The Shed, or perhaps, make Another Shed.