You may get the CNG kit installed on the petrol variant of the car aftermarket but it is recommended to be installed by the authorized service center so that there is no lapse in the car warranty.
It would be unfair to give a verdict here as the resale value of any vehicle would depend on certain factors such as the physical condition, kilometers driven, number of owners, engine's health, etc. Moreover, you may click on the following link and fill in the required details in order to check out an estimated value of your car: Valuation of Used Cars
For this, we would suggest you walk into the nearest dealership as they will be the better people to assist you. You can click on the following link to see the details of the nearest dealership and selecting your city accordingly - Car Showrooms.
