when did coure become a "heavy car" it shudnt be any more heavier than the imported 660cc.s.
A proper bolt to bolt swap should be a good option. You can even use the car on petrol but fitting an EFI engine requires certain modifications to the fuel tank/line etcs. Petrol mialeage should be better than your old carby engine.
If you are putting in 660, you should just drop the option of CNG.