I don't suggest the addition of a CNG, and that on a re-badged Lexus. CNG is rarely available in this country.
If mileage is the issue, do not use dual a/c, use only front ducts. Get the car checked up about this millage issue by someone whom is an expert on luxury car checkups, not by these random street workshops.
But if you seriously want a CNG, why not buy a small family saloon? Don't ruin this executive car, man. Enjoy it's luxury
I personally prefer the word 'small family saloon' over a 'compact sedan'