Are you able to test the speedo with another motor? It might not be the motor.
Have you stored it in the shed or garage where it might have got condensation in it? I've seen someone blow a speedo after getting the car out of the shed and switching it on after a few months
If you have a multimeter disconnect the motor and the battery and check the resistance between the phases and the battery supply. If one is low/shorted then that's where the problem lies.