The best place to get CNG installed is CNG Solutions (Near Akbar Chowk Faisal Town), I got my VITZ 2004 converted from there. They have experts who successfully installed CNG kits on alot of vehicles including SUVs.
I have consulted them and according to the owner (Sohaib Ahmed) VITZ 2005 onward model has good performance on CNG same as previous generation/models (1999-2004).
He's a nice guy and I am sure he will guide, his number is: 0333-4250670