oil galley plugs in 123 engine?

Have you "fished" the location where the pressure gauge screws into the block (just below the filter platform), as El Toro suggested previously? Was there oil present?

Does the engine run (are you running it without oil pressure or just cranking it)? If you are just cranking it, are the valves opening and closing (take valve cover off and look at rocker arms, are they moving?)? The oil pump is driven by the cam shaft. If the cam shaft is not turning for some reason, the pump is not pumping either....

What about the oil pump float (pickup)? Is there something blocking the oil going from the float into the actual pump?

I would drain the oil and remove the oil pan and pull the pump. Then I would use an air hose and blow out all those oil passages with air pressure.
If the shop removed the plugs I would ask the shop
if they reinstalled the plugs after cleaning the block. I wouldn't run the engine until you find the problem as you can damage the new bearings and crankshaft without any oil. Hal
PS: Make sure that pickup tube on the oil pump
is submerged in oil.

When you primed the pump, did the drive gear on the pump appear loose in any way? Any tolerance issues at the top of pump or bottom at the gear mesh (gear mesh coverplate & gear mesh, pump body and coverplate gasket, etc.? Was the pump rebuilt along with the engine? Is there something wrong with the pressure relief valve within the oil pump itself, is it opening prematurely?

What type of tractor (C, SC, 200, 230, 240, etc.)is this C123 bolted to?

Assuming the obvious (correct weight oil and oil level, not bleeding oil from anywhere especially the rear bell housing area), it sounds like some kind of galley issue. May be the machine shop forgot to put in the front galley plug.

Without the front plug, the oil would just flow towards the front of the engine and dump itself into the govenor gear/cam shaft gear/crank shaft area without building up much pressure (no resistance) Then run back into the oil pan. Unfortunately the plug is behind the cam shaft gear and not easy to see (I do not remember if you can see it when the cam is turned). Eitherway this basically means taking a significant amount of the engine apart.

Thinking out loud, do you have a way to pump oil? If it is missing the front plug, I wonder if you drop the oil pan, remove the oil pump and pump oil manually through the galley direct the hose toward the front of the engine, will it flood out the front of the engine? If you pump oil in from the pressure gauge port (without removing pump or turning the engine) will oil flow out the front of the engine?

If the rear plug was missing, there would be oil coming out of the bell housing area.

When the problem is found, please let me know what it was.