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First of all, did the "failed" oil pump trash the crank or anything else? If so, you are probably out of luck, as 310's haven't been made since, I believe, the 1960's. If the original owner said it no longer built pressure, I'd suspect he let it run low on oil and the hydraulic unloader valve then prevented the pump from operating correctly. If he ran it like this for awhile, it might not be good news. On the other hand, the valves may have needed replacing, and he didn't do that either.
I only ever worked on one Quincy 310 in 11 years of wrenching, but they are basically the same as a 308 with bigger cylinders. There may be a couple of things wrong here. First, the oil pump may not be priming. Some Quincys are terrible for this, and it seems hit or miss whether they will ever be reliable, even if they do work sometimes. I would suspect that if this is the case, the pump has worn its internal dimensions to the point where it can no longer create pressure. However, lets assume that as a worst case scenario for now.
I believe that your compressor has the old style vane pump. You can tell right away if it is, since is has a thin hexagonal plate on the back end covering the oil pump rotor and vanes. If this is the case, there may be several points of failure:
1. The vanes are worn out and no longer provide a good seal to the ring they run inside.
2. the ring itself is worn out
3. The compressor is running in opposite direction to the orientation of the oil pump. These vane type pumps are supposed to be self-reversing, i.e, depending on which way the compressor is running, the ring that the vanes run inside will rotate with the rotor for a few degrees and open up appropriate oil ports for that direction of rotation. This sometimes fails to happen, and there is then no oil pressure. If the pump is equipped with a hydraulic unloader valve ( a small square, brass valve in the side of the crankcase with and air line coming from the tank, through it, to the cylinder head unloaders) it will send an unloading signal to the head, preventing it from pumping. You can manually rotate this ring by hand with a hammer and small punch, after removing the hexagonal pump cover plate. Just rotate it as the rotor would if it was turning in the direction of rotation. It doesn't take much force, so if it doesn't move, don't beat the heck out of it, it's just a small roll pin in there. You should be able to move it with your fingertips.
Check to see if there are springs behind each vane. If not, the vanes will not make adequate contact with the ring and oil pressure will not come up. There is also an oring on the shaft that comes out of the oil pump. If this has failed, there will be no seal there, and oil pressure will drop. There may be failed orings in the pressure regulator. It is the screw and locknut arrangement coming straight out of the crankcase near the top of the oil pump. Try adjusting the nut inwards to see if any pressure develops. It is a simple ballbearing on a seat with a spring to set pressure. There may be some crud preventing the ball from seating. You may also have faulty seals in the hydraulic unloader valve that is preventing it from shutting. If this is the case, an air signal will be constantly sent to the unloaders and the pump will not produce any air. There is a cup seal and an X-ring in there, nothing major. The internal oil filter screen may be plugged. There may be an air leak in the oil pump housing preventing priming. there is a small setscrew plug in the passage leading down from the oil pump on the back of the compressor. Make sure it is not leaking air in there (rare fault). That's all I can think of right now. Oil pump failures are rare. I'd check all possible control problems first.
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