Dr Sajid!
Just keep things simple... Remember that Pakistan is one of the worst countries to buy used cars in... reasons as follows;
1- Terrible road conditions.
2- No guarantee of original fuel( patrol as well as CNG)
3- No guarantee if the last owner used genuine parts.
Therefore it is great that you are going for new Corrolla or Civic.
Please keep in mind, if the new Corrolla as well as Civic does not come with company fitted CNG kit, there must be a reason to that... After all they are not small companies but multi billion dollar industry... if they dont have CNG kit installed then these cars are designed to be used on Patrol only. When you will alter the original setting of company, you will have problems for sure.
Therefore, if you are interested to use a car on CNG, just find original company fitted CNG cars. Suck as Liana LXi etc. Otherwise just go according to best fuel efficiency of patrol cars in Pakistan local market;
1- Suzuki Liana
2- Honda City
3- Honda Civic
Corolla is towards the end of the list (got this off some magazine).