I thought about the o-rings but I was worried they would be to thick and not pliable enough to be stretched around the pulleys.  Its worth a shot.. something is better than nothing and i can get a range of sizes pretty cheaply from the local hardware store.  The only other idea I had was to try square cassette belts from electronics in the same manner, get a range of sizes and experiment til i get one that bites both pulleys.  Have you repaired one of these belt drives and used an o ring with success?

I have pulled out the christmas cart from the attic in an attempt to fix why it was placed there to begin with   The axle gears which is a normal issue with this cart were split and an easy fix. 
Where I am falling short is with the drive belt and motor.  Because it sat so long the drive belt dried and is holding an oval shape which no longer bites so I need to replace the belt.  Can anyone give me the size belt that was used.  Im guessing anywhere between 17-20mm.. its impossible for me to measure exactly since its stretched and no longer holds its round shape.  Since I have to take apart the electric motor to actually replace the belt I was considering updating the motor as well.  There are plenty of micro rc motors out there but am hesitant without knowing any specs of the original.  Can anyone help me with the OE electric motor specs?  RPM, voltage, etc.....  Thanks for any advice/info available.