I will not advise you to install CNG in your corolla for initial 2-3 years at least, as it will be voiding your car warranty, decreasing your car resale value, destroying suspensions and engine.
It will cost you around 30k -45K , depending on the parts and cylinder you intend to use.
A 55Kg CNG cylinder will give you 100-120Km in city with a fill up of 420-450Rs, in petrol you can do 60-80Km.
CNG will eat up at least 20HP from the 88HP available.
The future of CNG seems to be not very bright in Pakistan, and especially in Punjab.
The fuel saving would not be more then 20-30%, as new corolla has already a very good fuel consumption and it is 1.3L.