First remove the upper sump plug to be sure the supply pump is working. If the case is full above this plug the supply is working. Now try filling the return line with oil to prime the pump. If that doesn't work check the line to the sump valve for cracks and loose fittings. Then check the sump valve and pickup tube.
If all that didn't fix it remove the bottom cover on the pump if it is worn machine it flat ( I use a piece of glass with sandpaper glued to it with feather edge adhiesive) Now check the gears they should be the same height as the case so that the gasket will set the end clearance. If they are worn a little you can seal it with a little locktite plastic gasket and run with out a gasket. I have a 741 that I did this to 20 years ago and is still working. If they are too loose most of the larger Indian suppliers will have them.