fitting CNg kits to your car ads weight, if you plan on sticking on petrol for the time being you are jsut going to be lowering your fuel efficiency by adding weight so you might as well wait till CNG becomes affordable (if ever) to fit a kit.
however now might be a great time to buy CNG kits since people are removing them from their cars. with the demise of CNG and entire industry has collapsed overnight (local CNG fitter)