Check the flywheel key. it may have sheared. Usually from not getting nut tightened to spec. This is assuming you have checked all of the electrical connections that may have been tampered with when engine was swapped out.
It could be a simple loose battery cable, ground wire to block, spark plug wire, electrical plug to the stator. The list is endless without you first checking the basics over. It could have melted down a piston, but you need to check things over to find out.