So basically, this entire process is only worthwhile if the cost of conversion is less than converting to petrol engine and then to CNG and secondly, if the availability of CNG in the long run is guaranteed.
As things are going on right now, neither of these is true. If you still want to convert you should sell the diesel engine, get a petrol engine and then convert to CNG. Unfortunately, there are rumors that during winter there will be no CNG available.
and yes as Xulfiqar & Porche mentioned there are kits that blend CNG and diesel. Such kits can also increase the hp of the engine considerably while at the same increase the mileage of the engine. However, I would love if someone can point out their availability in Pakistan.