Starting a Cold engine and reving it hard even on its Original fuel (Liquid) is bad, then doing it on alternate fuel (gas) Must be Very bad.
I personally believe that running 1 km on your cold engine is worst then running a 100 kms on a warm engine.
And on CNG, the friction will definitely be more=more damage.
Best is to Never run your car cold (3 to 5 minutes of warm up atleast, when temperature needle comes to 1/4th atleast, dont rev it, just give it time to warm up itself, if it has choke then pull the choke to half until it gets hot).
Never start your cold car on CNG.
Never make your radiator fan Direct.