Anycar can work on CNG you just have to install the correct equipment designed for that particular engine.
My experience.
Corolla's with 2E, 4E, 5A, 4A, 7A, 3ZZ works very well on CNG with no issues.
Suzuki, Alto, Cultus, Mehran , Bolan work well on CNG. (Liana and Swift owners give mix feed back)
Coure and Charade engine also work fine.
Honda Civic known to work fine on CNG but they give kind of bad avg so unless ur living in Potohar area its not very useful.
Honda CITY IDSI and old ones work fine on CNG.
CNG on CVT Can work but in the end it can damage your gear, A friend is using CNG in his steermatic from last 4 years he is happy.
Baki who ever tells you CNG is this and that i am using CNG in my car's from last 5 years and I never had a single prob created by CNG. My Current car on CNG was last tuned in Dec 2011...
If you can afford Petrol there is nothing better than this but if you cant than CNG is a good option.