We had the same problem and took the gear box apart and found a company in San Pedro- Long Beach California
that fixed breaking material into the units. Then we had to careful put them on a lathe to get them to the correct thickness. We also replaced some of the bearings at the same time.
The biggest problem was having to lift
the engine forward to get the gear box out as there was only a couple of inches of movement on the prop shaft. The companion way is not over the engine so had to build a lifting rig.
Found the engine mounts were falling apart and replaced them.
Also had the fuel pump
failed and replaced that.
Recently had problems with head
gaskets which we are replacing and at same time we are having the fuel
It is a 1977 engine so I cannot complain. The basic engine is solid so I hope that it will now work for a while.