@malik011: what kind of setup u presently have in ur khyber, if it is injector in carb then i suggest, live with it, u need to spend fairly good amount of money to make it look alike cultus' setup. e.g. air cleaner box modification, carb hole filling/new carb etc. btw, 120km/hr with A/C is good while running on CNG, considering its just 1000 cc. u might need to tune up ur car more precisely and to ur taste rather than bringing in all new setup.
btw, check 2ndry throttle of ur carb, is it working? thats why u r missing top end performance.