It requires an arbour press and a few other things we don't have.
I personally don't feel comfortable replacing the shaft, but I don't see the need on Binky. You need it in situations where the motor is attached to a different bulkhead or mount to that of the weapon so that the shaft wont bend if the two twist. If Binky ever twists to that point... that will be the least of our worries as the motor and weapon are attached to the same body.
The shaft is 6mm all the way down anyway. if we go up to 8mm then I doubt the pulley will have enough material left to put the grub screw in anyway.