Hmm. If you know someone with a lathe, you can have them drill a hole in the center of the bolt, slightly larger than the shaft on the motor, then glue the bolt to the motor.
You can try to drill it yourself, but it needs to be pretty dang close to center of the bolt, and doing it by hand will most likely not do the job.
You may also be able to use a nut, and then some sort of reducer on the motor side. Nut with reducer glues to motor, the long bolt screws into the other side of the nut.
Maybe use that coupling bolt in that fashion?