No need to dismantle it if you’re just putting a longer spacer between the wheel and motor.

The issue is the rotor sliding off from the inside bearing. Most of the time, this happens when someone revs the motor too fast without bracing the rotor. Centrifugal forces pushes/slides the rotor out if there is no bearings or spacers to keep it in place.

Once you put the wheels (which has the bearings and spacers inside it) and the wheel-to-motor spacer on to the axle and tighten it with the axle nut, then the rotor does not push/slide out because there is something preventing it from doing so.

On rare occasions, the wheel-to-motor spacer may be too short and not able to push everything towards the center, then a longer one will solve this. Another part that helps to push the rotor is the wheel itself via the wheel adapters. If you do not have heat shrinks on the teeth/prongs of the adapters, then there can be gaps which can also prevent from pushing everything towards the center of the truck.