Is this the style unit you are working on?
If so, the missing key isn’t exactly a key, but a round pin, #25. It’s quite small so may have gotten lost on disassembly or may have rusted away. It fits into the square keyway machined in the eccentric and is a bit proud so it will jam up on the flat area on the prop shaft.
A piece of small stainless rod or a piece of drill bit will work just fine. Only problem is the piece of drill bit will rust away over time.