Sir! Please ye zulm iss gari pay matt karein... I also own a Fielder but it's 1500 cc. I'm quite happy to keep it on petrol because firstly, the petrol prices have gone down, and secondly, CNG is unavailable 3 days of the week (and it's being heard that this winter the shortage shall become even more severe).
So my sincere advice to you is to either make your use of the car limited, or get a 1500 cc one.