Originally I said this was “a Perkins 4-801 diesel.  Rated at 50hp but from what I have read more likely to deliver 35hp – which is somewhat academic while there are bits missing.”

This I think is where the water pump should be.  No, its actually where the Diesel Injector Pump should be!

But then I found the engine identification plate (which had fallen off the engine and was on the table)

This identifies the engine as a 4.236 made in 1994 – nominally 65hp and probably delivering 60hp.  Whether this is a replacement for the 4.801 or not I don’t know but it’s a much better size of engine for the boat.  The next question is whether the identification plate really belongs to this engine – I think it does – there is a place that clearly had a plate of exactly this size and the photos I can find look like this engine so I think it is accurate.

In turn this leads to the question of the missing part – I now think this where the fuel injection pump should fit and leads to the next question – which of the three possible types of pump is the engine set up to use?