ORIGINAL: proanti1
Are you talking about the TOP Plus 21 Flash? The only thing it shares with the Nova Flash car engines is the crank case. The crank, sleeve, carburetor, and head button is marine specific. Someone may have put a marine head on a Nova Flash, but that does not make it a marine engines...

Off the top of my head, the only marine engines that are truly just car engines with marine heads are either RTR, or the OS 15CV-RM, or some older K&B engines. Take a look and OPS, those are very specific to marine, and timed to run with marine tuned pipes.