The swap will not be economical & neither an easy task.
1. You will be needing a 6 digit amount
2. You will be required to buy complete engine with transmission, electrical wiring & an ECU
3. You will be required to register the new engine number on the registration book.
last but not least, the performance MAY not be as 'powerful' as you anticipate & car value will remain un-changed
the economical way is to rebuild the engine with maximum original parts. Pay attention with extra care to clean cooling lines of engine block head and flush radiator, upon assembly place back the thermostat valve, rewire the cooling fans to work on automatic. Break-in (500KMs) engine run on petrol, the switch to CNG - in the meantime also get CNG cylinder tested by HDIP Karachi.
( cng related recommendation is subject to CNG Kit used in the car)