Don't know the boat but if it's typical of designs of that era (1960's) then the engine may be in the middle of the boat, just forward of the main sliding hatch
If so a few hours with a ruler should give the information that you need.
In general it's possible that the engine and gearbox
could be unbolted from their mounts, slid backwards and then lifted out with a crane. At least one of the persons involved should have experience in removing an engine from a boat.
At this stage I'd suggest caution. My experience has been that once it becomes necessary to remove an engine from a boat rebuilding is going to be near as expensive as a new engine. Diesels are very expensive to rebuild and rebuilding an old engine with an unknown past involves risk.