78cc and 86/87cc both are same. Have ridden both and no difference.
The Bore is 47mm and the Stroke is 49.5mm.
This 49.5 mm stroke is what they share with CG125 as well, no wonder they are little pocket rockets. CG's bore is 56.5mm btw
A friend of mine had his chonda's (Union Star) bore increased to 50mm and he installed CD100's piston. He has been getting excellent response. He also added 125's CDI unit, plug coil, carburetor with free flow airfilter and sprockets size of 38/15
We both went to Nathia gali on it this Eid and i must say i was really impressed with it's climbing power.