Virtually every part is available, which is nice!
If all is needed in yours is a syncro...and you are taking the box out anyway, you may as well take yours out, take it apart and have a good look at it it...if that is all it needs then easy job. You could always fit the Series 3 syncro etc stuff into your box regardless, and then leave the sale cylinder etc as it is. Chances are if you buy a box, there will be something wroong with it that the owner does not know about! I would never had guessed mine had so many bits missing inside!