If you cant find a belt you can buy a length of it with jointing glue made for projectors,and make one up.A man on the web Bristol area I think sells it but you will have to search a bit as I dont have an address.Just search" belts for projectors "or similar-kit was less than a fiver in 2008.Good luck
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix film projectors.