Yes, the 20 is no problem. If you check the manual, the service life is probably down to 17 or so. There is some play there.
But the 813 is the length. In fact, it measured out to a clean 32 inches!
Remember, this is a proprietary belt made for KYMCO. That is why you can not reference it anyplace.
The SYM is the same way. It is like 919, also made by Bando, and guys have tried belts that were "close", no joy.