Totota 3L Turbo, its 4cyl 3000 cc and super powerful.
All the Pathans in Balakot area and Kagahn, Naran, Swat and Gilgit where these jeeps are used extensively use a Toyota 3B engine which is 4cyl 3600 cc.
For an economical result and normal road use you can even use the Toyota 2L engine used in Hiace which comes both in 4cyl 2400 cc and 4cyl 2800 cc
The best and most powerful engine is the Nissan Patrol 1990-92 model which is 6cyl 4200 cc, Its gonna be expensive but then you jeep is going to be the thoughest and meanest in the next Gawadar Rally.