As per customs act, it is illegal. People do it and the lucky one's are still driving such vehicles without any problem. But once they are being held by the authorities, they have tp pay the body tax. And that's why hundreds of people have paid it. My friend hassan_mirza9 is right that parts can be fixed without any issue once you have the receipts and another fact is that we have "noze cut" vehicles being imported in containers and sold at all big cities of pakistan but again i would say, AS PER THE LAW, IT'S ILLEGAL and should be avoided.
Basically the problem lies in customs. At the same time, we have non custom paid vehicles being sold around everywhere, especially the parts, and at the same time they confiscate the vehicles. Unfortunately if you are caught, what so ever receipts, documents, evidences you have to prove that it was just by putting together the PARTS in your vehicle, they are not going to listen and will ask you to go to the court. And no matter how wise you in explaining/ arguing, that will know that it's a total body swap. "Jis ka kaam usi ko saajhay". Custom people know every chalaaki, which we think of. My last advice would be to stay away from such vehicles.