Sir
my car is on cng from past 15 years!i got my car on cng in 1995 when there was only 1 cng station in islamabad which was known as kandan cng in industrial area!nothing happens to the engine if the car is properly maintained and all types of filters are changed on time!
Follow that advice and try to run on petrol in a month or two, it is good for every engine and car.
For people having carburettor, it is easy to pour some petrol in Carb and engine will start in no time in a single self even if its not ran on petrol for a year or two, provided that the fuel pump and lines are working, fuel pumps are too expensive nowadays, take good care of it, some people and mechanics use to disconnect fuel pump and block fuel lines which is not good at all.
Regards