Yeah, that is a bit too thin. 12 + (4/2) = 14mm which would be a problem. I would get a 12T sprocket with a 12mm bore since that should have a 16mm wide boss. You'll lose a litte bit on the reduction then (45 / 12 = 3.75:1) but assuming you are still using the 11T sprocket on the scoother motor, you'd want a 24T spocket to compensate (24 / 11 = 2.18:1, so 2.18 * 3.75 = ~8.18:1).