Hmm your car was already past its life so if you install cng to already weak engine and than it starts to break down, You should not blame CNG for it.
I am using CNG cars for 2 years. and this one from 8 Months i only get the oil change.
India is also going to use CNG in there cars and now i heard UAE also got plans.
So in future you will see CNG Vehicles all over the world. If you can afford petrol there is nothing like that.