I'm fairly sure the engine was formerly attached to a self propelled lawnmower SB. The cylinder head is cast in unit with the cylinder barrel and yes it is a 'round head' engine of around 98cc. The intake manifold is nickle plated brass of an astonishing small internal diameter which is a feature of these engines. The engine also has a simple but robust centrifugal clutch.
(Not my bike in the picture)
A chap here in New Zealand built himself a very nice bike around one of these engines.
I guess I could make some adjustments to the bike so it would better fit me SB. It certainly is a pretty little thing with its metallic gold paint. Something tells me that I'd definitely make the neighbour's eyes go big should I head out of the driveway riding on it
Money can't buy happiness, but it can buy chocolate which is almost the same thing
Home of the Kotanga Tramway.