I had a 12HP (Yanmar???) POS in my old 30' Hunter
. Yours may not be the same, mine had raw water cooling
and the piston stroke was horizontal. I had to redesign the motor
mounts to minimize vibration.
Anyway, my POS failed, saltwater got into the combustion chamber either via the head gasket
or through the corroded cast iron. I soaked the piston with kerosine for weeks, could not free the piston. The guy I gave the engine to hammered the piston to pieces in order to clear the cylinder walls. Think he eventually junked it.
Suggest that you NOT spend money
to repair especially if it is raw water
cooled. Consider searching for a good replacement that will physically fit where teh current
engine resides. Most likely you will need new motor
mounts, and a new shaft.
Word of caution with motor mounts! These things are not like screw drivers which are all essentially the same and found on everybody's shelf. They are engineered to specific operating conditions. For example, the boat's forward/reverse thrust is subjected to the mounts. So a mount with an adjustable height should be avoided because the horizontal thrust vector applied to the mount varies with the height of which the thrust vector is applied. Better to shim under the mounts for height adjustments.
The mounts must be designed to meet vertical thrusts along with horizontal thrusts. They are also designed for specific load frequencies (vibrations). And it gets better! They have their own resonances and you want them to be as low frequencies as possible.