This is not a normal failure mode. Normal failure is usually compression that decreases over time until it's so low that it won't start.

His mechanic, apparently without doing any testing, says it the oil pump, and he needs a new engine? Did the oil pressure drop before it lost power? Did the oil level light come on? If not, why suspect the oil pump? Time for a new mechanic?

The neighbor got it running, but couldn't keep it running, so he thinks it needs "ONLY" a CRANKSHAFT and BEARINGS? Why would he think that?

Loss of power is usually because of a loss of fuel. If it will start, but not keep running, it's probably because it's not getting enough fuel, or not getting the fuel at the right time.

Loss of oil pressure is usually accompanied by an engine that stops turning, and will not turn, or if it does turn, it will knock very loud.

There is not a lot of information to work with yet, we would need to know more.

How many miles on this engine?
Were the fuel filters ever changed?
What work, if any, was done before this incident?
A complete description of it 'lost power' including what the gauges were reading, how long the engine ran after the smoke, etc.

I'm sure we'll all try our best to help.

Best Regards,
Jim