Hi Ty, I have checked my Scott Bonnar info but unfortunately most of which covers cylinder mowers, but there is reference to a rotary called the "Spin-Cut" being produced in 1958. It doesn't mention your model 22, but the timeline seems about right. Scott Bonnar also used JAP engines on the 14" Model 40 "Terrace" mower and they were first introduced in 1960.
It is a shame that the fuel cap is missing, these are sometimes hard to find for vintage British engines.
