Hi Bryan,
You're right in there being a sensor - there's an auto-stop mechanism, which triggers a solenoid to disengage the play, if a reed switch is not closed. This reed switch is held closed by the electromagnetic force created by a rotating ferrite wheel in close proximity to the reed switch.
You need more than 1 belt, and member Dillen can supply the belts for you.
You will also have to soften/renew the rubber on the idler wheels - best done by holding a tissue with alcohol against the rubber wheels, while the idlers are rotating.
Once the rubber is soft again, there will be sufficient friction against all the plastic wheels to rotate the ferrite wheel, and keep the reed switch closed - I have spares if you need!
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!