Well, you've told us what specific engine concerns you. You've told us you have a tatty copy of the users manual. You've told us that you are concerned about "keeping it running", specifically you said "...and I know jack about how to keep one running."
So why, precisely, do you think it won't keep running? Once it fires up, this beast, like every other small marine diesel, requires you to do something quite specific and quite drastic to MAKE IT STOP!!.
Like interrupt its fuel
supply. Every small marine diesel has a little lever, that when you pull it, does just that. Additionally there is a switch or button on your instrument panel that does the same thing by quite pedestrian electro-mechanical means.
But you probably meant "how do I prevent it from wearing out" or some such. The answer is that you can't. Just perform the maintenance
that is specified in the user's manual. If you do that, the beastie should be good for at least 5K "engine hours", and the number of hours it has already run should be indicated by an "hour meter" that is either a separate instrument on your instrument panel or a component of your tachometer.
Do the math: If you run the beast 40 hours a week, which would be exceptionally high usage, 5K hours equates to 125 weeks or more than 2 years. If your usage is a more typical 10 hours a week you may expect about 10 years of use of the beast.
For Sunday sailors that only use their engines to push the boat
in and out of their marinas
and put maybe 50 hours on the engine in a year. the beast's life expectancy is a hundred years!
So just do your maintenance
according to the tatty manual in which there will be a maintenance schedule set out in table form. It even specifies what grades of oil
to use and what the ID numbers of the fuel
The "sore spot" with little diesels in boats is usually the cooling
system because saltwater is corrosive. But your manual will tell you about that also.
So there you have it. It's that simple. Just follow the manual.
All the best.