Bell and Howell TQ II Repair

Hello,
Im not sure which forum to post this on, so I hope this will do
Just wondering if anyone knows how to replace the worm gear on a Bell and Howell Filmosound TQ II, and where to find this worm gear?

I found a YouTube video on this exact subject. I also recall there was someone on eBay selling the worm gear. Hopefully they are still available.