Ideally, you would go to a Show with the chuck and search stands until you find a key that fits.
But these are not normal times, anywhere in the world.
In the past, there have been one or two instances, where the pinion on the chuck key fits the chuck, but the spigot is too large. In these cases, the chuck key has been put into the lathe and the spigot turned down to a snug fit in the hole in the chuck body.
May not be absolutely correct, but it does the job!
Howard