I think those have the 2 speed tranny? I have heard that some tranny parts are non existent, really hard to find? might be untrue, maybe someone will chime in.
a lot of them have a second gear problem, but you can buy a kit, you would have to tear down the hole motor to install it, i never liked the QA 50 bikes, 1970's z50's & ct70's are much better bikes i think,