Pistons of CD 200 are just slight bigger then of CD 70 So it woudnt fit in ur bikes cylinder. Other options can be GS 125/150 & CG 125 wich may work wid some alteration.
BTW why dont u use its genuine piston?
Another option can be the piston of Honda CB 180 which can increase power of engine as well provided if u manage to fit that. But i think the parts of CB 180 are rarely available only in Karachi.