CNG is a real technical nightmare,It has been used in New Zealand for over 30 years,it's NOT Propane!.To obtain the gas ,you either have to have access to a CNG supply station or you can have a pump in your home garage connected to the town gas supply.This can take several hours to fill so you connect the car in the evening and leave it to fill by itself.In the morning the car will be filled and will have shut itself off from the pump.

In this city ( Brisnyland) we also have Volgren Busses running on CNG,the range is only about 400kms and if the level is getting down ,the bus must be returned to a depot and refilled,its a major headache if it runs empty.

The ONLY set up with either CNG or LPgas ( propane today is to have Injection.You will actually have more Horsepower with the injection than gasoline.
The number of injection suppliers is grownig by the week.
http://www.blazegas.com/kits2.html
http://www.lpg2000.com/products.html
http://www.classiccartimes.co.uk/features/Workshop/LPG/Feature.asp?ArdSI=2a2f396967c7706d5d172ac64e0a16e0