Peter,
I would suspect an automotive origin rather than marine. Marine would be more likely to have a water cooled exhaust manifold, and the clutch looks automotive. There doesn't appear to be any provision for a self starter, and the long snout at the front of the crankshaft does look like the mounting for a permanently installed crank handle. This would be awkward to use in this position in a typical mounting for a small marine engine.
What are the threads on the fasteners on the main parts of the engine, and are the hexes on the nuts Whitworth sizes, American type inch dimensions, or metric? This can often give a clue to the country or area of origin. E.g. prior to WW1 some German machinery used Whitworth threads, but with metric hexes on the bolts and nuts - conversely, early Morris engines were metric, but with Whitworth hexes on the bolt heads and nuts. Things like magnetos and carburetters can be changed over the years, but items like cylinder head bolts, main and big end bearing bolts etc are rarely changed from standard.
Lack of a self starter and the fitting of the priming cocks would probably place it no later than early to mid twenties if it is of automotive origin.
Though there are heaps of exceptions, cast-in inlet and exhaust manifolds seem to be more common on early continental engines than those built in the UK and USA.
Good luck with the identification.
Regards,
Frank
