Well never go for Margalla 1000cc.
Reasons.
1 - Its ok on Petrol but on CNG it will be too slow.
2 - If you use A/C with CNG than well it will be even hard to cross bycycle.
1000cc margalla will consume more CNG than 1300 CC simply bcoz engine will need to work harder on higher RPM to drag that sedan body.
Now consumption. Mehran will give u a bit better Milage but Margalla in good condition i feel should give you around 2 to 2.5 Rs per KM which is good enough.
I had charade sold that and bought Corolla 1989 so far i am feeling hardly any difference in the gas consumption even though charade is hatch back 1000cc and this is 1300 CC.
Other options u have try and find 1988 Corolla if you are not intrested in Margalla otherwise go for Margalla.