I believe Cen do a bag of bits that should sort you out. I dont have the part number, but Im sure it includes a couple of grub screws, optional springs (for the Rally, ST, and MT setups - make sure you use the right one), and a few steel balls that sit behind the spring are in there.

Am I imagining things guys? Or was there such a thing available? I cant find it now... Aha - here it is... I think... G84302-12. Info taken from here - www.toyhaus.com/images/schemas/2speedtran.jpg

Failing that - a 2nd hand 2 speed (or one from a new, broken for parts truck), can be had for not a lot of money from ebay, with the advantage of spare/optional spurs, topshaft etc included.