When an engine sits for a long period of time the oil tends to become thick and sludgy. The cylinder walls can run dry as well as the bearing surfaces. Any oil in galleys and passages and the pump will also become sludgy. In order to help flush this gunk out, you need to perform a few oil changes after the initial sart-up.
Seeing as how you had head gasket failure prior to the engine sitting, the coolant may have been introduced into the cylinders and crankcase, ultimately the oil. This frothy mix if crap can wreak havoc on the pump, bearings and any other rotating or pump items.
Head gasket failure may have been happening little by little, prior to the ultimate blow, and this small leaks may have introduced coolant into the engine for some time prior to it being obvious. This crappy mix would have been pumped through the engine coating and filling every surface where oil should be. This mix, as stated before, can kill those bearings and clog whatever it passes through.
Seeing as how several oil changes did not take place after heads were fixed, this funk may have been left in the engine.
Another note is that if the mechanic performing the work was not careful and old sealant and gasket material may have been dropped into the cylinder walls or oil galleys. These bits of crap can clog the works up. This may be the trash in the oil pan.
Option 1:
Change the pump and then repeatedly change the oil for the next month. (Weekly). After that, as long as pressure remains stable and good, resort back to your normal service intervals. Also, do a compression and leak down test.
Option 2:
Pull the motor and go through it with a fine toothed comb. Replace bearings and gaskets and anything else that needs it.