Stock length is def 729mm for agility with 12" wheels. I ordered a new belt not long ago, 729-18-30, but it doesnt feel right so I refitted my old stock belt for now.
Im sure the 18mm wide is too wide for me, top speed was limited cos it wont go right down into the bottom of the rear pulley, and it just felt rough. When I compared new belt to old stock belt, the new one was a good 2mm wider. I ordered a 729-17-30, once it arrives I will try that, hopefully it will work well.
Some sites list a50 belt as 18mm wide, some say 17.7mm, others 17.5mm. I emailed Kymco the other day to ask them exactly which size they recommend, no answer yet but will let you know what they tell me.
729mm long IS he correct length tho.