Use whichever method you feel most comfortable with, and most importantly, safest.
I recently bought several couplers from Skycraft surplus, which is a great store here in Orlando, that were multiple sizes. However, I had no difficulty in drilling out one end or the other, or even both ends, to allow it to fit as needed.Ive only seen the couplers with up to 10mm bore unfortunatley. Have you actually seen an 8 to 13mm?
You may think that is a good idea but most motors have too much speed and not enough torque to directly drive a wheel that moves a car, a cart or whatever. Make sure your motor has enough torque for the job before you incest in the mechanical design effort.