Start preparing the list of parts which are missing on your Unimog, e.g. front headlights, traupulin frame, fenders, engine hood etc. Body parts would be the most hard to find items considering the speed of scrappers who are cutting down these Unimogs. Best place to buy parts would be the Unimog killer (the auction guy) from where you or your friend have picked up the Unimog for you. They are everywhere, you just to need to put some effort and time to locate them in Karachi!
The engine of Unimog 421 and Mercedes 240 D (123) car are same ecept few changes and 24 V electric system surrounding the engine (starter motor, genset, guages etc.). The Engine re-built parts wouldnt be an issue in Karachi. I have recently searched an Army Spare Parts supplier in Karachi for a friend (Haider Shah) in Karachi who is also restoring a Unimog. Pls note down the following address. This supplier was probably out of town and expected to return by 20th Feb'12
Malik Autos
Office # 17, 1st Floor, Dikasha Chamber
Waheed Murad Road(near Bandar road)
I am hopeful that you will get new brake wheel clinders, hub seals, clutch cylinder, master cylinder, gearboot, torque tube boot, odometers, gauges, bearings, diesel & engine filters, all engine parts etc. This Unimogs are no longer in Military service to try your best to get a bargain.
You will get an idea about cost once your list is ready, be prepared for it, and go step by step, first grab the parts which are harder to find!