let me get this straight - you have a 660cc engine which you bought for cheap commuting, then also added CNG.. hmm interesting. So now you are thinking of changing the engine - why not just use it on petrol. By the time you will justify the cost of the swap it will have become too expensive.
Its like a person I met in Karachi who had bought a used prius.
He installed CNG in it. So the driving cycle was.
EM ----- Engine --- CNG ---- brakes ----- EM ----- engine ------ CNG --- brakes all day long.
EM = electric motor.