A good (meaning available) freelance boat diesel mechanic is hard to find anywhere nowadays, and when you need him next he will have just retired. Alas.
In MDR, CC Marine is a professional outfit with a good rep. At the moment, that's where I'd have to go.
But as Ray says, diesels are typically worked on by owners who gradually learn the simple basics by asking for help--right here. Then just plunging in. It's really hard to make a diesel engine problem worse and 90 percent of problems have simple solutions ( which becomes apparent only in retrospect, of course).
Also, hiring somebody is often unsatisfying because they don't show up, or because they don't want to work in cramped old sailboats with ancient wiring or because they know that you don't want to pay for everything a responsible mechanic thinks needs to be done. But mostly it's unsatisfying because when it gets fixed, you never really know what went wrong, or how it got fixed, and have to live in fear it will happen again. Which is no fun and kills confidence in the boat.
We have old boats and old engines. They need our personal attention.
(Wait--Atomic 4, not diesel. I think the above applies to that gas engine, too. Our Atomic 4 owners will know to what extent)