Diesels actually need "less" to run that a gas engine.
1)fuel (at the right time)
2)compression
3) air, lots of air.
ok, lets make some assumptions.
Lets assume you've used fresh diesel, and changed the fuel filters.
Lets also assume, you bled the system, and used the "manual engine primer" might be a rubber squeezie thing in the fuel line, or, a lever mounted on the fuel pump.
Lets also assume you have flowing fuel. (the fuel valve open?) supply is able to be pumped via pressure to the injector, so check and make sure you're getting fuel past the pump.
because the wd-40 test kinda sorta worked ( a couple of firings....IS working)
lets continue down that path.
stop at a truckstop or auto supply, buy some starting fluid.
squirt a LITTLE. no more than a pfft, pfft. Dear god don't treat it like your wife and her white rain, lots of metal bits will come unattached in a violent fashion.
If you have compression, the motor SHOULD fire and run a second or two, or longer if the fuel supply is flowing well.
If it dies, repeat. (remember, just a "pfft" of starting fluid.)
It could take a number of these starts until she fires off on her own and continues to run. The engine will not like starting fluid. Its a violent eruption. Again, don't use too much.
If after a half a dozen of these attempts, and its still hasn't wheezed along, you need to look at other places.
Look first at air supply, is it free flowing? or is there a shut off plate under the aircleaner (on some engines, this is how you shut them down, by cutting off the air supply)
Then look at the fuel supply circuit.
Is fuel getting ... first to the fuel pump.
If so, is fuel getting out of the fuel pump and up to the injector. (unfasten the line to the injector,) fuel should spurt out of that line. when you're turning over the engine. If not have the pump checked.
If so, then, remover the injector, have it checked.
Theres no logical reason to rebuild a motor that doesn't need it.
the front main seal should be able to be replaced in situ. Worry about that after you get it running. It may "loosen" and seal after it gets some warm oil next to it for awhile.
"cold start lever"
this allows some of the compression to escape in cooler ambient weather. if the ambient is say, over 60F, leave it closed. (have a strong battery and make sure you don't work the starter too hard.)
This "should" help.