Small Stuart ???

I don't think it's ST either.

The carb is not typical ST - a strong strand on all their engines - but odd in different ways.

The mating face of the upper crankcase half is not a good match for the upper face of the lower half. ST were very good at casting work as befits their model making background.

The petrol tank is a later edition. Look at the amount of exposed thread on the inside of the nuts which hold it down.

I agree that it looks vaguely like an early P or N series, but is too small & I think the likenesses are merely generic in small two strokes.

I also agree that I wish it was mine, too! However, the ABC beckons & I have a unexpectedly free weekend.

I am fairly sure it is a Stuart Turner Lightweight petrol engine, they were produced as casting kits years ago. The carb differences could be the result of someone's own engineering methods to modify/improve Stuarts original design. There was correspondence about these engines in the SEM letters pages and Discoveries section 1992ish. I will see if I can dig some info out at some point. Regards David