Oil to the shaft is supplied through the rear shaft stand, comes up through an oil port in the head.
Loosen the rocker shaft bolts just enough for the rear support to raise off the head. Start the engine and see if oil comes out under the stand. If it does, the oil hole in the rocker shaft is clogged or the shaft was assembled wrong.
If no oil comes up through the head the rear main may have spun or something is wrong in the lower end of the engine.
Since the rockers have run dry, the bushings and shaft are probably bad. The shaft and bushings can be bought, but the bushings will need to be reamed to fit the shaft. Much easier to buy a rebuilt shaft/rocker assy.