During the few times I was not in the field, and got to spend time in the huge motor pool areas at Ft. Hood, Texas, I personally witnessed and discussed with mechanics and drivers this very subject. Being raised in So. California in the 60's and 70's, My Dad bought and sold Corvettes and muscle cars as a hobby before it was even cool, so I had a bunch of knowledge about "matching number" "Vettes !
I saw multiple units gutted and motors swapped with no regard to where they came from originally, as long as all the pieces were there ! Ran around with a duty driver for a Lt.Colonel that owned his own M151A1 ( Liked the split glass ) and he was able to have the guys upgrade the motor, paint it, whatever, and was actually a P.O.V. !
My buddy also told me stories of drivers trading favors to the Pool guys to get a better running engine, and the reverse, putting thier damaged engine in the vehicle of a back-up unit and so on ! Have to remember that these folks had the ability and equipment to do in a fraction of the time what would take us many more hours, if not days !
There used to be quite a bit of "good ol' boy" fun, and am sure that it still exists today, but thinking it was a lot easier to run rampant before the days of computers, barcode scanners and all the goodies used now !
For this very reason, I have always assumed that nothing on an MV is "original", at least to that particular vehicle ! Inspection, Inspection, Inspection !