Is the piece stamped steel as It seems? It has a light area just above the edge contacting the magnet, that looks like it was under a nut or bolt head or washer. Maybe a piece of the windage tray or the oil pickup? What is the history of the engine? Could be someone put some baffling in the pan that came apart or lost the piece when it was assembled. Not much in the way of stamped steel in there to begin with.

The starter should fill the hole unless you have a mini starter which leaves a gap.

I don't know if you will get the pan off without pulling the engine, I tried and failed.