Yes he can put CNG in a Mira 660 CC. But it will not be a practical approach. Reason CNG to me works best when you have a engine that is over powered for a car and you install CNG that reduces its torque making ability by not less than 20%.
The 660CC engine that is already over worked in a body frame more suited for at least a 1000cc engine (i feel)will have to do more work when on CNG. This extra burden slowly not only damage this tiny engine it will also be a pain to drive especially if you want to use a/c.
This Car must give you at least 16 KMPL on petrol if its not than you must look for issues. Change its plug/ Fuel Filter/ Get Injectors Clean ( A guy do Ultrasonic cleaning in lahore) but only ultrasonic cleaning Ustaad cleaning is big NO NO. Take to toyota if they can do a scan and help you.