I could be wrong, but isn’t the area where the key goes to hold the flywheel onto the crank shaft. So, it looks like your flywheel is missing the key/pin.
Since they are pretty tough to get off, I don’t know that someone makes and sells that specific part on a regular basis. But you can contact DeFelice Engineering to see if they can help you out. http://www.defeliceenginc.com The guys a DeFelice are experienced machinists who specialize in C & P restorations, among other things. Their work is just beautiful. So, if they haven’t scrapped the part from another machine, they might be able to custom make the piece for you.