1999 Dodge Dakota Oil Pressure Issue

My truck is in great condition and has regular maintenace i.E. Oil changes, etc. Recently the check gauges light came on then oil pressure drop to 0, then knocking noise. I quickly turned it off. Then had at garge for repair which they were unable to reproduce so on my way home from picking it up the same thing happend except this time I had 8 miles to go when it occured. I ran the truck with knocking noise about 5 miles at low speed. It now is in the shop they were able to reproduce the problem after letting it set for sometime running. They are insisting I need a new engine because of the knocking noise with pressure drops. Can I just replace the oil pump and as long as the pressure is fine it will be okay? When it is running and oil pressure is good there is no knocking per the mechanic. We are talking 900.00 vs. 6400.00 so just want to make sure I do need an engine or can get away with pump replacement. Thanks for the hlep!