I too am victim of aftermarket belt slop. I wonder if that is a real phrase?
One question seems to be underpinning a lot of the observations, What are the actual dimensions of a brand new Kymco, OEM belt? Several of the suggestions for replacement have close to a 40mm discrepancy in belt length and I've never seen what a factory supplied K 23100 measures out at. Perhaps one of the chronic failure problems is due to a wrong size being used? I've been dealing with a very good mechanical engineer in Michigan recently who is heavily involved in the motorcycle industry and his thoughts and experience in Snow Mobile and ATV drive lines was eye opening.
In essence he thought that a good aftermarket belt could be satisfactory IF the size was right. The problem for him was that most belt systems use a jockey wheel to accommodate the variable ratio adjustments. If there wasn't one on the machine, you were better off using the stock design.
These are just random thoughts offered to add to the mix. - However, if you do have a brand new belt, most of the old fashioned auto parts houses have a special belt measuring fixture / tool which will give the absolute number. It could be interesting to find out if all the stock belts are the same length.
I have a 2006 P250 which has about 18,000 KM and am shopping for an Exciting 250 specifically for touring back roads in the Upper Midwest and central Canada. I also am the caretaker of another eleven motorcycles dating back to the late 1930's but prefer to ride the P 250 for everyday riding ... the old bikes are normally ridden for short distances as they are not too reliable ...