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Would like to add my experience in building 139QMB engines over the last 12 years. The largest kit that will install and that is made is 85cc at 52mm. The 100cc kits that are sold all over the internet are simply not good and most are 70cc at 47mm. There is no such thing as 100cc for the 139QMB engines. I have seen many a so called 100cc kit that were install come to my shop for a new piston and the 47mm fit perfect. NCY is the only distributor that has 52mm kits that come with the correct head. Naraku makes one but the exhaust port is no larger than stock so performance is limited. The correct built CVT with a good kit from 70cc  -85cc that will be 47mm, 50mm or 52mm will last a long time. The correct CVT parts are a NOT worn out clutch and bell, proper torque spring for rider along with a variator that id not restricted which all stock ones are and you can see the restriction. The CDI in all 139QMB engines is limited to 7500RPM once again CDIs are not easy for Kymco which all are DC activated that I have come across. NCY does make the best AC pulse one for the 139. A performance exhaust is always needed to get the full performance out of the big bore kit.

Now in two stroke machines 52mm is 90cc or 107cc or 110cc. The scooters that have these stock are some Yamaha and others only in Asia. The way to measure CC's of your engine is TDC and then how much liquid is in the chamber. The Genuine Roughhouse uses s 52mm piston at 107cc

There are a few engines that are 85cc that can be bought in the states. Some of these are on ATVs from Kawasaki and others these are the 152QMB, in the model number for GY6 is the piston size hence 139, 152, 157 and so on. Have not seen a 90cc for some time the ones I have seen are in Linhai atv's for about 7 years now due to I sold plenty of them.

I am more than glad to help.