as these are rather basic machines any older diesel mechanic can work on them young blokes can too if their willing to learn(the basics of engines haven't changed in 100 years we just have better materials and tolerances today and the electronics). some of the tooling for the special jobs is still available. and yes get a copy of the three manuals as others have suggested.

the dealer mechanics will work on this gear for a fee of course and that fee aint cheap. for parts try the dealer if you have a part number they can help you so a parts book is a must have.(note there are separate manuals and parts books for the engine and attachment such as blades and hydraulics units)
the challenge is finding a diesel mechanic willing to have a look.

i would start by removing the manifolds clean out any dirt and remove the injectors so you can pour diesel down each cyl followed by draining the engine oils checking for signs of water in oil if no water that's a good sign refill with fresh 15w40 diesel mineral oil. if you cant remove the injectors drain the rad and pull the heads remove any dirt found and add diesel let it soak for a few days top up the diesel as needed using a block of wood and hammer give each piston a tap. if you can get the engine turning over thats half the job.

Mr trainzkid88,
Thanks for the good info.
5U4848

TK88 is right. These old tractors were meant to be worked on. Get yourself the proper manuals for the engine, chassis and any attachments you might have such as blade/hydraulics;
My first attempt at reviving a D4 would not have been possible without the books. It made life much easier when there was something to read and sometimes a photo or diagram to reference. I was so deep into the tractor, failure was not an option, and I was determined to NOT ask for another person to make it run for me. Well, I succeeded, and dummy me, it got me hooked at that point. Hahahaha.
Another note about un-sticking the engine; sometimes the lifter bodies get stuck as well, so be sure to soak with oil very well in addition to everything else.
Best of luck to you, and hope to see/hear it running soon. JM