Oh what a mess.
First your engine number appears to be 31S977-0008-B1. Poorly stamped.
31S977-0008-B1 IPLFirst apparently was ran without oil. Probably caused by a blown gasket.
What you think is a mystery key is not a key. It part of the camshaft ACR that has broken off so you will need a new camshaft.
Clean up the crankshaft using muriatic acid to remove the aluminum transfer. This stuff is nasty to use especially in high moisture areas. Always perform this outdoor with a good wind flow. It can take your breath away. Alos rinse with water to neutralize and oil the surfaces to prevent flash rusting.