The stock belt is Bando 23100 KEC4 9000, and I just installed one from an Agility 125cc, it is same same.
I tried an 821 20 30, too long, 815 20 30 too long. an 803 20 30 would work, but the actual size may be the 781 20 30 mentioned.
Check with Carlos at Scooter Assasins from Taiwan on ebay, he has most Kymco belts.