Here is my opinion, first off there are no 80cc Chinese 2 strokes. Having said that what you have is an option for a 49cc or a 69cc. They both have close to the same amount of power in general. The 49cc has less torque but will rev higher. The 69cc has more torque but will not rev as high. So you can get them to be about the same if you gear them accordingly. In my opinion the 69cc has more of an upside, in that, because it is larger displacement with a little tweaking you can make it produce much more power than the 49cc.
Also either engine will die if you try and start them from a dead stop using the clutch with the normal gearing.