As the others have mentioned, you will be SOL if the previous owner has already thrown out the old drive. If they haven't, you will need to get it from them and extract the drive key from it.
The new drive DOES NOT have to be the same make/model as the old one (unless you are just going to swap the logic boards, an easier fix).
You can "Spoof" the old firmware and key to a new drive, essentially making the 360 think your new Benq drive is really the old Samsung (or whatever make/model it used to be).
Also speaking from experience, over 100 drives replaced.