I have had great success with modern V belt on my belt drive machines - in C size/section which is 7/8" (22mm or so)
When I can, I use endless belts. They are cheap and last for ever - almost - but can only be used on machines where the rear frame can be unbolted to fit it in the first place. No big deal: you might need to do it once every 5 years if you ride it a lot. The only tricky bit is to get the length exactly right; on a Douggie neither the rear axle nor the gearbox pulley is adjustable. The length required will depend on your gearbox position (tension the primary chain first!) and the pulleys fitted. Here in Australia the belts come in 1/2" (1.27cm) increments, and are still readily available at transmission specialists for less than $50 or so. Fit and forget.
The green Varifit(?) belt (pre-punched holes every 1/2") and a joiner (with a hinge in it - don't use the two flat plates they try to give you) is OK but delaminates after a season or so. We bought a spool of expensive black belt from the US and it was much, much better. It was designed to be used with a joiner - if you cut an endless belt and try to fit a joiner it often just pulls out.
Leon