Cathy
Check out Hobby and Engineering Supplies in QLD as they have a vast range, including SDP-SI belts and pulleys, at good prices and shipping charges. I have delt with them for a few things over the years and found them very good to deal with. They usually have most things in stock.
http://www.hobbyparts.com.au/
From the home page, scroll down to almost the end and look for
Timming Belts and Pulleys [headings are in alphabetical listing] and I am sure you will find the correct parts for the modifcations.
Chris