Since the car was registered in Karachi, Their is no way it was registered without the orignal file. So its either with the bank, With the excise dept or you misplaced it.
In case you are sure you misplaced it, and have the NOC from bank, You would now have to get a FIR and a add published in the local news paper, and get the orignal news paper add and FIR to the excise dept to issue you a duplicate file.
In any case do contact some agent they can help you charging reasonable fee. Do contact your relatives or someone you know who got a car registered recently. or else their are several agents who can get it done for you after asking for required documents.