Most Yanmarengine model designations are on an engine data plate, on the front or the top of the engine. Some older engines have the data plate on the adapter plate between the engine and the gearbox .
The boat is a 1978 Watkins 27.
I've not yet seen the boat, nor does the previous owner know its history.
The oil fill and belt arrangement should be good indicators, but I've not been able to match it up with pictures from a Seloc manual nor pics on the web.
edit: The dip stick looks to be directly below the oil fill.
No major problems with anything as they are pretty simple engines. Parts are resonable as far as cost goes and are available if you can get someone who knows how to look to see what part numbers have changed and there are a few. I used the boom and a small chain fall to hoist the motor out. I have easy access to my engine thankfully. The only special tool was one to pull the cylinder sleeve out of the block made from some ready rod and some bar stock. Easy job but very dirty. Diesel engine oil is as bad as or worse than 5200 for getting on everything......m
I must go down to the sea again.........