What are the main manufacturers of diesel engines for boats?

not enough information
to tow anything , there are regulated engine power requirements , braking capacity, electric break away brakes on the trailer, weight on the tow bar, boat width, boat length
All the requirement can be found at the vehicle registry office or main roads department

With a boat 44 years old, your boat engines may have been replaced or at minimum, overhauled. You might resubmit your question with some easy information you can add:

Gasoline or diesel engines?
Many were Crusader engines & you can usually see the name like this on the top portion of your engines.
You can see what color the valve covers are (blue, black, etc.).
You can see if they are V-8 engines if they have two long valve covers on top.
Most engines have a name plate on them (yours may be gone if original engines). Any mechanic you run into might find it quickly or recognize the engines.

Enjoy your boating. Pete

Yes! Diesel hybrid engines have been available since late 2008 and boat experts predict that they will become much more popular in the next few years.

The main manufacturers of gasoline engines for boats are: Crusader Engines, Flagship Marine, Ilmor, Indmar Marine, Marine Power, Mercury Marine, PCM Marine Engines, Teague Custom Marine, and Vovlo Penta. For more information on these manufactures, check out this website: http://marinefuel.com/marine-engine-manufacturers-gas-or-gasoline/ and http://www.discoverboating.com/buying/types/engines.aspx?id=79

The main benefit of diesel engine (compared to gas engines) is that it consumes less fuel and much cheaper (especially now a days with gas prices rising). In addition, diesel fuel is non-explosive and its engine last for a relatively long period of time.

The main benefit of gas engines (over diesel) for boats is that their engines are much cheaper to both buy and maintain/repair.

The main types of engines when it comes to boats are: steam engine, gas turbine, diesel engine, steam turbine, and gasoline engine.

Yes there is. Saw one at a boat show and I believe it was also Swedish. You may want to look for used replacements on the web. I do not know where you live, but there are boats on the Great lakes with low hours because of the short boating season. There are also victims of hurricanes with good engines down South.

I owned one diesel engine. I would say the engine siphoned the diesel after it was shut off.
The throttle controls may not be closed. The injectors were tested for their opening and spraying, but were they tested for closing and sealing when off? Diesel fuel lines maintain a vacuum or they get air bubbles. Cooling cylinders create a draw right into the crankcase. Last idea-crankcase vent clogged

Try checking the fuel injectors to see if they are leaking.