LPG Installation will cost U around Rs. 22,000/-
Whereas Margalla Fuel Tank @ 3000 + Carburator Service@ 1500 + Fuel Pump@ 1500
Restoring it on Petrol will cost U 6000/- approx.
Furthermore U'll loose all the empty space in your trunk after CNG+LPG Cylinder. Plus U'll put another fixed load of 30-40kg on the rear suspension.
Like sahhil_22 said "in the longer run you will be spending all that amount on engine maintenance which you saved from NOT using petrol as secondary fuel."
These engines are designed to run on Petrol and U want to ruin it by using it on alternate fuel. Bad idea man