Actually the torque on petrol is also less.Comapred to 7AFE at low ends.
That is why people who wants to have low end torque with 4AGE engine build 7AGE. (combination of 4age and 7AFE). However it still still more than 2C engine.
If you can find some good electrician and CNG person then u can surely use it wih CNG.But it will be little more complicated but doable.
Why people are against CNG in 4AGE is due to the fact that 4AGE is a performance engine and requires more attention and maintenance than 4AFE or 7 AFE. Thats why people advice against it that for an already complicated engine why add another level of complication.
Also CNG is against the spirirt of 4AGE. Every owner that I have seen with 4AGE is always trying to add a little more boost to it by some modification so adding the CNG is against that normal theory of modification.
But at the end of the day its your car and you can try anything that you like with the car.