I'm at a loss.  What possible difference would it make to the buyer
whether it was the original engine or not unless you are telling him
it is?  If you don't know, then don't say it's original, just say you
don't know.  I don't think it would make any difference in the value
of the car unless there is something special about that particular car
that depends on it being the original engine, like if it was
originally owned by Richard Petty or something.  The MVD doesn't care
what engine block is in it, all the care about is whether the thing
passes the emissions test.

   You can find the casting stamp on the bottom of the block, just above the
starter.  If there's a lot of dirt and grime it will be hard to read.  If it's clean,
it will still be hard to read.  So be diliogent in your search.
    The casting number will not match the VIN on your car.  It will tell you when
the block was made, though.  I don't live in AZ, never have.  But in all the states I
have lived in, nobody checks the block number to register a vehicle.