This is quite interesting to read about modification of the most famous car of Pakistan "Mehran".
If you could fit in Charade GTti 1.0L Turbo engine or Vitz 1.0L VVTi engine, performance will be modified. But I am not sure that if the light body of Mehran could tolerate such heavy weight engine and would fall apart.
Else, get the 660cc Daihatsu Mira (JDM) or Suzuki Alto (JDM) engines, they would certainly more fuel efficient and perform better than 800cc obselete engine of Mehran.