Another Mystery (small)

I have had this engine for many years, but have no idea of its identity. I believe my Father made it from castings, using his Drummond round-bed lathe. It appears to have a 1.5" bore x 2" stroke and is about 14" long overall, with twin 7" flywheels. It has an atmospheric inlet valve, an 18mm plug and what I believe is a gas carburettor, although I am sure that my Father ran it on petrol. It was originally set up to belt drive a bright red (WW1?) aircraft dynamo, with a bullet shaped end cover (the latter sadly has gone missing) Please contact: Diccon Roberts