Please do not follow the above advice
You probably need a new head gasket, and possibly a new injector. Might as well replace the intake and exhaust gaskets too. You can replace them yourself, or with help from a mechanic. A parts list and manual with torque values can be downloaded online. You will want to clean out the carbon on the head and piston also.
It's an easy job that I did in just 2 or 3 hours. You'll need a torque wrench, which you can borrow from some auto parts stores. Don't use ether in a diesel, or it may break rings and crack pistons.
. If it aint broke dont fix it!
OK It MAY be broke but there is no evidence of that yet.
It is unlikely to be the head gasket if it had blown there would almost certainly be other symptoms esp. overheating.
It could be the injector but again there would be other symptoms, in nearly all cases, black smoke under load usually near smokescreen quantities.
First priniciples. Are you cranking for 20 seconds at full throttle when you are trying to start it.
If you are - check your RPM cranking . [white bar on crankshaft pulley and a model helicopter blade tach. is one way. ] if low battery and supply wiring to starter should be checked. for voltage drop.
If cranking RM is ok then check compression dry and wet. Best to get a diesel tech to do this. He may suggest valves or rings as the culprit and maybe just maybe the head gasket but that would be rare.
BUT it is a 43 year old engine and when you start spannering you may be opening a VERY expensive can of worms, if it runs well, without overheating or smoking I would just buy a spare can of easy start. If it aint broke dont fix it!
Oh yes worked on my first diesel about 50 years ago and still have grease under my nails.