POINT1 :Typically speaking - carbs are much better than efi's. reason EFI is just a simpler solution not a better product, just like a split a/c you can install anywhere, but in cooling it doesn't match a good solid long lasting built window ac. However in pakistani market cars, have mostly come with average/poor quality carbs. A good built carb or a substitute double/triple barrell carb /sports spec carb can beat an EFI hands down.
POINT2 : GT B13 AND GT B15 are now hard to find. You can try and order from Sher Shah in Karachi. Even if you install go for higher power B15. Agree completely that you will need to install better front shocks and modify braking for the B15. However both these are old products and you will struggle in maintaining them. PSM (computer) is also old fashioned and upgrades to software is almost impossible locally. Parts not available. Newer small cars like city or even much smaller 600/800 cc reconidionted imports have better computer control module & software than on these engines. - However main problem is no part is available now in junkyard or in market and if you happen to need any engine part - you will be in trouble.
If you decide to buy one, make sure you take guarantee of gearbox differential noise/ play in gears (ghuccha). Another option is to buy from UK/US online re-sellers of engines and other parts. they will arrange international shipping. - These engines will be available with them and other options as well. One such retailer - market.autopartsfair.com.
POINT3 :There is another option available . There is a japan swift (khyber model) available in junkyard market. It is similar looking but not same to marghalla - It is inline 4 cyl. full TTC carb with (with piston and also option of simple butterfly) auto choke. this is 1.3 Litr. The engine is donkey engine - use it for daily drive and it can give local khyber or cultus very good boost. The G. Box has good ratios and is much faster than the one in local cultus. You can install this one and also install simple yet effective upgrades like ignition coil, core 2 wires and Bosch plugs. Spend money on upgrading engine header and muffler pipe to give yoursel a boost and can try even a free flow carb air filter or build a custom made air intake scoop , however in carb the scoop does not give as much boost as in Efi. but still can give some gains. Rest you can spend on body and alloys and converting to 4 wheel disc / ABS booster. This engine should be trouble free comparatively.
POINT 4 : If you are really itching for power , look for an engine in 1.6- 2.0 efi, DOHC (turbo or naturally aspirated) category, and modify engine housing to install it. it will be easier for you to find parts for a newer model engine.