This might not be the kind of response you are looking for, but may be the only response anyone has.
I bought two of my Changs from a wealthy auto dealership owner. He had imported 16 Changs as he thought they were interesting (cleaimed to have even sold one to Whoopee Goldberg). But he sold all of them off as he said he couldn't keep them running. This guy owns multiple large dealerships around town and has one location that is several acres with multiple brand dealerships located on the same "campus". He also owns dealerships in Florida. And needless to say, these dealerships have repair facilities with mechanics, so this guy didn't have a lack of guys to work on his bikes. But it does seem that many mechanics can only do what they have seen done and many will only work on certain type vehicles.
A Chang isn't a particularly difficult machine to work on. It is old technology from simpler times. And parts are available.
So my suggestion is go look for your mechanic in your mirror. As one of my old martial arts instructors used to say, everyone is born a baby and must learn to walk. You can find information on repair on these scooters. You can ask questions on this list. And there is another list you can ask on, you just must make sure that you always say good things about the paying sponsors and never let anyone know when one gives you a bad deal or you will be banished and insulted. And in the end, you will be much better off when you know your bike and you can maintain and repair it yourself and that makes riding all that much more rewarding.