You can either go for a Landi Renzo or Tomasetto Kit. I am not sure of the prices, as they seem to fluctuate every now and then. For a Vti, i would suggest that you get a 60W cylinder, which would initially give you around about 60-70km in a full tank (Rs 330 approx). Once the car gets used to CNG the mileage can go upto 100km without AC if you don't floor it.
BTW, the engine performance will eventually deteriotate as it runs the engine dry. I believe Friends CNG would be your best bet in terms of reliability and service. But you will have to pay a certain premium in terms of price, when compared to the market.....id say a couple thousand.