How about a bushing under the coupling ? I assume there is a shoulder on that shaft, so whatever you put in there should be seated against the shoulder. It seems like you've got enough engagement on the key, you just need to make sure the coupler doesn't shift towards the motor during operation. So basically you need a spacer. ID is known, OD can roughly match such of the coupler, and so the only thing you need to figure out is how much thickness you need for it.