FWIW, we have been using off the shelf belts to fix these. The belt is T5 pitch, 12.5mm wide 66 0r 67 teeth. We drill a hole in one of the teeth using a micro drill bit of 0.75mm diameter, and then press in the original long pin. In addition, a collar is used on the pin to set the forward distance the belt can move towards the siding gantry. The reason is it's near impossible to get a belt with a ridge in the middle as a guide, but it's not strictly required and with some attention an off the shelf belt can be used. While this is a 16mm wide example, at worst it hangs off the front edge of the pulleys, but may not then need the collar https://www.amazon.com/Gates-T...Length/dp/B00CJFSLMI
I was hoping I had pictures of the belt in place, but it was done at a friends house. All I could find was measuring the pitch of the original belt and then the video of it working after the replacement belt was installed.