It is very easy. First thing to do is to check how fast can a CD70 go. Then take a marker and write 120 at that certain point on the speedometer. and Bamn!!! next time you go fast, CD70 will touch 120. Lol
Jokes aside, a friend of mine owns a modified 1985 CD70 which he used for a very long tour of several borders of Pakistan in early 2000's. He is not into street racing and I am not sure if that bike can clock 120 km/h, but I have seen that bike easily keep up with CG125s even when carrying a pillion rider. Next time he is around, I shall ask him the details.