Tesla (the company that makes CNG fueling/dispensing equipment) has already done these conversions for commercial fleet customers (think banks, courier services etc.) on a test basis.
They will retrofit your factory stock bike with a scaled down CNG kit for less than Rs 15,000. The cylinder has a capacity of approximately 5-7 water liters and is installed directly under the seat (nervous people need not apply for this conversion!).
The remainder of the kit is a scaled down version of a passenger car kit.
Running cost to user is approx 25-33% to that of petrol.
I don't personally know of anyone riding one of these bikes. I saw one of them on display at an industrial exhibition and discussed the project with one of the Tesla staff. They started up the bike and it ran fine on the stand. Of course, power output will be diminished, compared to petrol; but cost savings will be significant for the end user. I've spoken with a few people and they all say that they would jump at the chance to get the conversion done, once it is commonly available.
On a side note, I do know some people running CNG rickshaws and they love everything about them.