I don't know all the particulars, but you probably want to remove the flywheel cover and see if the flywheel is very rusty where it passes the magneto. I heard a story about a Briggs engine that had been stored under a tarp. had so much rust on the flywheel that when they triedto turn it over, it broke the magneto mounts off the block. If the wiring has been burnt, you need to check the wiring under there anyway.
And while you are in there, make sure the starter gear is not stuck in the flywheel.
If it has an electrical clutch, you want to make sure the clutch bearing has not seized. A seized bearing can mimic a stuck engine. The easiest way is to remove the clutch stop. Then try to turn it over. Just don't try to start it like that. Not only will it rip the wires from the clutch but it will probably be out of balance.
Make sure it has oil in it. I have seen several stored outside that somehow filled up with water. Probably due to missing or improperly installed air cleaners.
A squirt of oil in the cylinder through the sparkplug hole won't hurt. If you do get it started it will smoke.
I have also seen a valve stick so that the engine binds up when the cam lobe comes up against it. But that was on a Kohler engine. Just be aware that if you have part way rotation that could be a possibility.
Then there is always the possibility that some catastrophic failure caused it to seize up. Like running without oil.