plug the exhaust holes ,its not a 2.4 L bridgeport making 280hp @ 9000 RPM why they tried to make it have offshore exhaust makes me lol,see if ya can tap the holes and put some brass plugs in it ,if she pops up in reverse find the parts there probably reasonably from a outboard graveyard or hold it down with your other hand,how often are you in reverse ???,build up your transom to fit the 20 inch longshaft,the exhaust housing transfer , possibly different gearcase ,driveshaft lenghts ,labour involved is more than the engines worth ,id live with it or sell it and buy something more suitably to fit your craft