I am a CNG lover and always got all my Automatic cars on CNG (City/Optra/March/Gran Escudo(Japanese Jeep)/CL7/CL9/Harrier/Prado (my friend)) and always got it done through one guy who is in my view Master of this job and can tell you any car can have CNG. But even he said to me when i bought MarkX not to put CNG on this car. CNG is not good for MarkX, This guy has never failed in installing CNG in any car including (BMW & Merz) but failed to successfully install CNG in a MarkX.
So my suggestion to you will be don't put CNG in a MarkX and cuz i couldn't afford my MarkX on CNG so i sold it. BTW mine was also doing 5-6 in city
Rest is your choice man, if you want to take Risk like i took for my Harrier (3.0L) your choice it may work for u.