I'd put my money on a mower or garden tractor of some sort.
The front pushrod tube would block critical airflow in a bike application, and who would want to set those points when they'd be tucked in behind the front guard?
The real giveaway, though, is the port arrangement. Quite why you'd go to the trouble and expense of OHV, and then defeat the main advantage of it by using right angle ports, I do not know. New Imperial tried it once, and quickly abandoned it. So why do it? The one advantage that the right angle inlet tract would allow would be a close-fitting cowling around the engine, perhaps feeding around a fan on the flywheel.
Points ignition obviously needs a battery, yet there are no signs of a generator or any mechanism for starting. Bet they were somehow tied in with the transmission, an electric start. Any 350cc mower would be a beast, whatever it's from must have had a powered drive.
cheers,
JFerg