Maybe PM will tell us soon. If it's an OHV it brobably is a Bar/Buick/ auto conversion. If it's a propper marine engine it's almost certainly a Chrysler.
Chances of it being anything else is slim.
But if one happened to have a Chrysler water cooled exhaust manifold or a Bar manifold for a Buick it would'nt be hard to find a good car engine "in the days".
North Western Washington State USA