Pindi has good restoration mechanics. Stick to original engine. The original carb should give you around 9 KM / Liter economy (this is very good specially for a 2.4 Liter engine). You can further improve its performance by porting the head and replacing the exhaust with a free flow.
If you change the engine, then you have to change everything i.e. gearbox, differential, suspension, shafts, axles, hubs, brakes .... Some people say that replacing the engine with a 2C will be enough, but if you want a real experience then everything must be changed. Otherwise stick to original and enjoy your vehicle.