If you’re handy with a file Chris, you could make a woodruff key. Measure the size of the empty space, take a visit to your local stainless steel fabricator and ask them for an offcut as near as possible to the size your after, and file away until it fits.
Some thirty years back the woodruff key fitted to the shaft of the burner fan mechanism for our oil fired boiler at home disintegrated one winter. I went to the local supplier and he said I’d have to buy a complete new burner system at something approaching £300 plus fitting etc. I said no thanks, went home and eyed up a steel nail that was a bit bigger than the space it would have to fit, and less than an hour later we had a working boiler again. ‘Er indoors was quite pleased too !!
In your case the horse has already bolted, but I find the camera on my mobile phone to be very helpful as an aid memoir for things I would most likely forget, otherwise I always used the same safe place for small odds and bits I wanted to keep safely, and on my boat this was always a particular sectioned off space within the chart desk. Sometimes it can help to ask someone else to look for the missing item as a fresh pair of eyes can often see things that your own eyes are not recognising. Whenever I took the wheel off I would use some insulating tape to secure the key to the shaft.
