Call a mechanic last. Buy a book on troubleshooting marine diesels. Run the usual checks--it will make you an expert on your engine model, tank, filters, and hoses.
If you have normal-looking oil and coolant, and there isn't a deep pool of oil or diesel under the engine, and if your batteries are adequate, you can do everything a mechanic can do to see what's wrong, if anything.
In fact, tell the forum your symptoms and some member will recognize them and tell how he solved the problem.
A new engine is a great idea, and will probably include new fuel tank and panel as well. It's the last solution, not the first.
[Also: around here the big marine engine dealership says "we don;t really work on old Universal engines." Because the staff "hasn't been trained for it." That leaves you with freelance guys who tend to put you off or don't show up. I asked a marine industry friend why--don;t they need the work". He said, in effect:
"They don't know you. Customer says "it won't start." They arrive and see that the engine is neglected. They tell you the panel is a mess, the cooling system is clogged, the wiring is dangerous, the fuel tank is leaking, and that your batteries "probably" will not hold a charge from your obsolete shore-power charger (yeah, sir, the batteries are new but the old charger has destroyed them; and "probably" because charge-holding can't be determined with a glance) and gives a rough estimate of minimum work required.
Customer: "Just make it start so I can go sailing."
In the end, the good freelance mechanics have apparently found that just to tighten a ground wire, an hour's billing, isn't worth the drive and the conversation and the argument.]