I personally wouldnt recommend CNG on any vehicle, just LPG is as far as I would recommend, however, "IF" the 660cc engine has a turbo and "decides" to drink petrol like a 1300cc (while still acting like a 660cc) then I wouldnt blame someone if they wanted to save a few pennies by considering CNG. After all, the whole reason (as far as I understand) in buying a 660cc is to save a few pennies.
I never understood why Pakistan didnt move on to LPG fuel though. On Ferozpur rd, Lahore where it changes into Lahore Kasur rd I think after or around Atari Darbar area (after Qainchi) there are (on both sides) two LPG stations but both look like they have been abandoned and shut for a while!! What a shame. LPG has a better energy value, lasts a lot longer, becomes liquid at less than 10bars, and hence needs a lot lighter tanks.
My Octavia goes 250 to 300 MILES (around 480KM) in a 60 litre tank which really only holds around 54 litres of LPG at around 10 bar pressure. Not sure about the weight of the gas as its sold by litres in the UK, but I guess it cant be too difficult to work out weight when we know the pressure/density and the volume.
Really wish LPG makes its way in Pakistan and we can save our engines, and our precious gas reserves for using at home for cooking food !!