It doesn't move like your ex-mehran did? :S
Definetly something wrong with the engine, I have owned a Baleno Gti and that my friend is one of the fastest -if no the fastest- Paki asembled Suzuki.
There can be a number of reasons your Gti engine is performing worse then a mehran's, worn out spark plugs, ageold oil, bad CNG tuneup, faulty wiring, clutch and whatnot.
But since your saying the previous owner has replaced the 1.3L with a 1.6L the problem -Allah forbid- might be worse, the wrong ECU, wrong injectors, the wrong gearbox. I don't want to scare you but I have seen my fair share of bad swaps over the years, people save some money and go with their previous gearboxes when swapping engines, at times the wrong ECUs or wiring is used I have even seen weird hybrids with 1.6L blocks, 1.3L heads, 1.6L GearBoxes and stock ECUs.
Anyways, get your car properly checked, there certainly is something wrong, otherwise the GTi is the FASTEST Suzuki produced in Pakiland ever. Lets just hopes it's something as minor as a tuneup.