How to modify a engine with 100cc to 70cc without increasing the knocking tendency of the engine.can u give some ideas... regarding this.

Going from a 100 to a 70cc will require a sleve to reduce the diameter of the cylinder which will be determined by the stroke of the piston. Your cc's is the volume in your cylinder from top dead center to BDC.
If going from 70 to 100, what did you change?(piston, cylinder, carb/jetting, exhaust, manifold)?