As the good Dr Roper advises
Unless you have another reason, or you think the bearings are needing replaced I simply would not bother, or you are very bored over the Winter.
You could crab it remove box and check the clutch too.
Shimming the selector drum can make a significant difference to the quality of gear change (it will never be a Japanese or even Ducati box though)
If you are considering going into it you will probably need one special tool to get the input (Clutch) shaft nut off, but apart from that the rest can be done with normal tools
Want to know how or what you are getting into?
You'll get Pete's article on the box here and mine. Pete's is more technical, mine is more of an idiot's guide (as that what the author is).
http://www.thisoldtractor.com/guzzitech.dk/gb_en_complex-technical.htmI left it at that page because there is wealth of other articles in there that are simply brilliant, Pete's engine strip and build for instance are the stuff of gold dust.
Big thanks to Gregg Bender for continuing to host this site and give everybody free access to a great resource, which costs him cash for absolutely free
John