Yar, what are you all laughing at???
As for those serious enough, Yes the conversion can be done. THe engine which comes in Mehran is the F8B with 4 Forward and 1 Reverse Gear box. All the Suzuki "F" series engines have interchangeable gearboxes.
The 660cc you mentioned is the F6a and the newer version is k6a, both have 5 speed transmission. Both gear boxes can bolt on DIRECTLY to the F8B engine. The 5 speed gear boxes are good enough for upto 180km/h IF your engine can provide enough Torque.
If you want to know where and how to get that gear box, let me know and I will guide you. Cheerz!