The Agility 50 should do about 40-42mph or so on the speedometer with a standard weight driver. The older ones, from the around 2008-2012 were faster. There are a couple restrictions on the bike. The CDI has a rev limiter build into it that you can cut out or replace with an unlimited CDI. The other is the clutch boss. You need to change that part with an unrestricted one. That is the part that the front variator rides on. You can also make sure you have 6.5g roller weights, the 4 slides on the variator are in good condition, and there are not flat spots on the rollers(which you said you did not have.) You may also upjet the carb to a 82 or 85. There was also one year in which they put a restriction inside the intake manifold that needed to be popped out. You will have to check your laws on making modifications that may violate emissions or DOT guidelines in your state or country.
If you changed your belt, make sure it is a factory belt. The OEM belt cost more but it is excellent and last longer than most aftermarket belts so it pays for itself over time. It typically should be changed every 8-10k KM. They typically snapped at 12-14km. If you are using an aftermarket belt, that could be your problem as I have seen numerous belts that say they fit an agility, but they are not the right size. Aftermarket Agility 50 belts are harder to find than the popular chinese belt sizes so it is easy to get the wrong one. If the pulley got stuck, you should definitely clean it. That should not happen. KYMCO pulleys usually last the life of the bike. Check your tire pressure. Running low on pressure has a huge effect on speed and accelleration. And finally, if the bike sat, you could have a dirt dobber in your exhaust or a partially clogged carburetor.
Hope all that helped.
