I have seen a advertisement in 'Daily Times' Lahore edition about conversion of LC Cygnus into CNG; they say that if will be like petrol cuz they have some kind of chip etc and will not ruin the engine.
My advice to you is that never think of this kind of thing as it will spoil the whole party.
Frankly speaking if you cannot afford petrol then switch to some other car and if you can afford petrol of the elephant than you must go fot the 4.7 petrol version as it will give the real pleasure.
4.2 D & 4.7 P LC need to be "properly taken care of" and in the end there is not much difference of the expenditure.