I must say, I have never heard of an oil pump being worn out on an engine like yours, I wonder if your mechanic has tested the oil pump or have they just ASSumed, or guessed, that it Could be the cause of your problem?
How much time was spent? Was the oil pressure actually checked with a gauge, or is it just that because the light is coming on, the oil pump is responsible?
I believe you need to carry out more testing.
Plenty of mechanics ignore the fact that you must spend time diagnosing, and testing, before placing blame on a component which may or may not be faulty.
I would hate for you to invest in purchasing and installing a new oil pump, only to find the issue remains.