History lesson:
Kymco started as a factory, making engines for Honda, who is the inventor of the GY6 engine type.
At some point, Kymco made a brand of itself instead of just being a production for others, and soon started making their own motorcycles, possibly with the same quality control that they were used to, when they manufactured for Honda.
Kymco has been marketed very well in USA and in UK (and possibly in other countries, but not here) and soon became among the biggest producents in the whole world, as these are owned by themselves and not by car producents as many other big motorcycle companies are.
139QMA, 139QMB, 159QMA and others are just names of different subcategories under the GY6 type and the name tells a little about its specialities as the numbers of cylinders (1) and the bore (39) aswell as other measurments.
If you want the name of the engine instead of just the type, you'll just have to check what numbers and letters is cast on the engine.