Sir Pakistan shape Alto was discontinued in 2002, I recommend you go for the 6th gen HA24 Alto, if you are going with Japanese one.
Also please check that weather your car has optional ABS, Airbags (driver and/or passenger) or not, because apart of them, there is very little charm in a HA12 Alto.
That were very good principals, not practical though.
"Free flowing exhaust reduces back-pressure, yes, but it may or may not result in increased power and decreased fuel efficiency"
If they do not increase power or power curve then what is their purpose, Decoration or Sound?
Every free flow exhaust will increase power in HP, if implemented properly.
Now practically he will mess up his CAT converter and Oxygen senor both if he will try to do something with his Exhaust, especially on CNG.
Aftermarket Free flows are not recommended even by sports car manufacturers, unless they are done by approved tuners, let alone a Suzuki Kei car.
CNG needs a near perfect Air/Fuel Ratio, (especially in 1.5 Litres + cars), that is why mechanics block the air intake.
Now if the intake is factory fitted manifold and Exhaust is free flow, that is not good for both power and fuel efficiency, in other case, if he will install both a high performance air intake and free flow exhaust, he will also have to go for a richer air to fuel mix.
That will definitely increase the fuel consumption, weather he will be using any type of fuel.
The tuning of both intake and free flow on CNG is impossible.
Nearly all Ferrari cars since 550 Maranello uses variable back pressure exhaust, just to increase power and fuel economy is not the concern.