Ended up using 4mm of spacers for the motor-mount (5-6mm would work better; but then the motor mount plate would hit the spur-gear-holder), and 2mm of shims between the spur-gear-holder and motor-mount so it doesn't rub.
Thanks. I'll keep my eye-out on that center-shaft.
And, how many millimeters of spacers to make it "perfectly" centered with the aforesaid shaft?