Well the first and most important thing is you will face difficulty selling it after upgrading to EFI as people in Pakistan go for "genuine". They will buy your car quickly if it is in stock condition. But after doing jugaard of changing engine from carby to EFI you are going to face trouble in this section. (changing head assembly to same new assembly is a different matter as your car still stays in stock condition)
Second swap of this kind is always expensive. You need complete assembly along with complete wiring and ECU. (finding used wiring and ECU for mehran will be a tough job as EFI model is new in Pakistan. You will spend days and days in scrapyard just to find these two things) Placing ECU in car will be a mess. You will have to make special arrangements under car dashboard to place it. Other way is to just bind it with wire or put under carpet but that is not safe. I am also suspecting that your car may need exhaust upgrade as well. Making new exhaust is going to cost thousands of rupees (cultus carby to efi upgrade requires making new exhaust setup but I am not sure about Mehran)
Third there is no guarantee that you will get good engine from scrapyard. Seller might open its tappet cover for you and show you the internal condition of head but you will learn about condition of seals, piston rings and head gasket only when you put engine in car and fire it up. Then if you are unlucky you can only repent on wasting money.
So dude no matter how much you love your car just let it go and buy new. You won't face trouble with what company has made after research and with proper equipment.