I tried an online search,you probably did already, no great joy leapt from the page. There was some references at boatdiesel.com, which others here use, and reference to the engine as a 4/58 and 4/39. Also reference to replacing them with a 2.5L engine out of a Transit van. I`ve a vague recall of an English diesel truck called a "Tempest". More likely Tempest were a Ford engine marinizer, like Lehman. You could ask Brian Smith at American Diesel, the Lehman successors.
I doubt they use much fuel. Question is, keeping them running. Who did the owners servicing, there`s a starting point.
Island Gypsy 36 Europa "Doriana"