OK, let's start at the top:
1. The oil pump's primary rotor pulls oil from the tank and the delivers this under 60 PSI pressure to the filter, then to the long "log" on the back of the cylinders where the oil pressure switch is located. Then it goes to the top end and crankshaft.
2. After the crank bleeds its share out into the crankcase and the top end dribbles its share down the cam chain tunnel to the crankcase, it collects in the oil pan. The sump pump on the bottom of the same oil pump sucks this warm oil up into a second, lower pressure rotor. The ouput of this rotor is split 2 directions by a small wye that directs about 25% to the transmission and the rest back up to the tank for cooling and so the other rotor can get it again.
That's the whole system.
You have something blocking the wye in the pump, or else the screen on the bottom of the pump. Normally, the screen will prevent anything from getting pumped into the wye, causing it to get blocked.
BUT...if too much sealant was used on the cylinder reassembly, it has probably found its way into the screen on the bottom of the pump and blocked it. I have seen many poor mechanics do this sort of thing: the 750 engine requires NO sealant anywhere except between the crankcases, if the gasket surfaces are not damaged. Too many wrenches use it anyway, and far too much. This will ruin those crank bearings in short order, if allowed to continue.
You said the mechanic did not disassemble the bottom end: what work did they do? The only thing you can do to a top end, then, involves pulling head and/or cylinders and changing pistons and rings (or a bore job). Even changing a cam chain requires removal of the crankshaft (unless a riveted master link type chain is used). So, if the only work done was, for example, pulling cylinders for some reason and reinstalling them later, and lots of sealant was used (when none is required), chances are this is the source of the stuff that has blocked the oil pump screen. If there is that much stuff in there, it has probably also blocked the oil passages into the top end of the engine, which will destroy the cam next...these oil jets can be removed and their passages cleaned, but it means taking the engine apart again to do it.
I do too much "forensic reapir" on these engines, caused by today's mechanics...