Hugely long stroke, Colin and I've never seen such a long piston. I am truly astonished that you got even a fart out of it, much less a run. I'm assuming it was built
as a marine engine
or would you say the finned barrel of an aero engine
has been turned down to fit the water jacket
A very large shouldered top to the crankcase. it really is a mystery.
I can only suggest home made.
Edited to say it's 1.36 cc., so a bit of a nipper.