1) I agree that it's probably not a fuel/carb problem, but make sure the choke is open to make sure enough choke cleaner/starter fluid is getting through the carb.
2) How are you checking for spark? I wouldn't just trust a spark checker. From my experience, the problem is rarely the spark plug(s), but sometimes.....it is as simple as the spark plug.
3) Does it have compression on both sides? Are the the connecting rods broken? Or is a valve seat dislodged?
4) I've never seen a flywheel key sheered on a lawn tractor/zero turn.
5) I've seen very odd things caused by malfunctioning ignition switches.
6) Charge the battery or jump it. It might be just cranking too slow.