I was under the impression, from watching the video, that the motor runs. With it all opened like you have just reinstall the battery in the tender and re-connect the coupling wires.
Have you inspected the axle gear? If not, remove the axle wheel assembly and hold the white gear in one hand and try to spin the wheels. If the wheels turn then the gear is bad. Inspect the gear closely looking for cracks, long ways between the teeth. On another note those rubber traction tires look pretty worn, might be a good time to replace them now that you have it apart.
Have you inspected the axle gear? If not, remove the axle wheel assembly and hold the white gear in one hand and try to spin the wheels. If the wheels turn then the gear is bad. Inspect the gear closely looking for cracks, long ways between the teeth. On another note those rubber traction tires look pretty worn, might be a good time to replace them now that you have it apart.