Righto, sounds good on the spring. So just a new plunger then. If you run a wire brush up there on the end of your drill, you can check that the seat is clean enough for use. If it has significant rusting, you might need to send it to your machinist to get the seat cleaned up (or try with your twist drill bit of that size first). We're assuming that the gears are intact and turning but that's ok because you needed to do the relief valve anyway so hold off pulling off the pump until you've tested it with the new plunger. I actually have a hard nylon plunger in my 5U which I wouldn't have thought would have been enough to seal but apparently it's enough to run the tractor, even under power. Was surprised by that.

Maybe - is that serial number from the rear of the engine block, or from the rear of the transmission case above the hitch? If it's from the engine block, then Eddie's saying that the pump is not an original type for your engine, but it will fit, and that, unlike what I wrote, the plunger is steel. Since that's the case, can you inspect your steel plunger for its condition. If it's in good shape, then it might be reusable. It needs to provide a good seal against the seat so the pump can make pressure but it also needs to be free to move back against the spring to relieve pressure above a certain amount - I won't guess at what that is but I'm sure Eddie knows. You might be lucky and by removing the plunger and spring released some crud trapped in there. Take a really good look up in at the seat with a bright light.
If the serial on the engine is 4G something, then they may have fitted a whole 'nuther engine onto the transmission (if that's a go between a 4G and 7J - I don't know. It is between plenty of other series of tractors)
(A 4G is an earlier series of D4 than your 7J series. It's usual to refer to a series by the first two characters in the serial number. Yours is a 7J D4, mine is a 5U D2, with the full serial being 5U13753)

Great point Scott - that's happened more than once or twice. DC, let's see if what you have can be made workable easily. It might be just a matter of cleaning up the plunger and seat some. My 5U's situation is not flash but it makes enough pressure to run at full power so that's all I need. Might work out the same for you.