851D hard to start after sitting

I have a 1958 Powermaster 851D that I think is leaking fuel from the injector pump. The leak is hard to spot due to oil/fuel build up around the the pump but there is a puddle of diesel oil under the tractor (say about 8oz) after sitting for awhile. When I purchased the tractor just over a year ago it started very easily. Now it takes about a minute of cranking for it to start after sitting ( 4 hours or so). Once started it is fine for the rest of the day. The only thing that has changed is that I added Sea Foam motor treatment to the fuel tank just prior to this happening. Since the fuel tank is above the pump I am somewhat at a loss as to why the pump would lose prime like that, if that is what is going on here.

I do not feel comfortable using starting fluid with a diesel so I have not tried that. I have tried injecting diesel fuel into the air intake pipe (after the air filter), charged the battery, cleaned terminals and wire fittings etc. without any success.

Unfortunately I have not been able to get to the property much of late and have little time to diagnose this when I'm there. Just thought I would throw this out there for suggestions so I may have better luck next time.

You'll need to pull the pump off and have it resealed, mine had to same problem.

Sounds about right. There are not many diesel mechanics where the tractor is. Is this something I can do or is it more involved e.g. a complete disassembly? I'm good with my hands but that pump has a lot of parts!

It's a pretty simple job but you should get yourself a IT manual or copy of the factory service manual just for reference.
I that the injection lines loose at the head then remove them from the pump after it's off the tractor.