They are easy to rebuild but tedious. Professionals will do a good job too, but you can do it yourself. Best bit of advice is to get a piece of wood dowel slightly smaller than the layshaft and use it to hold the needle rollers in, and drive it out the front with the new layshaft when putting it back together. I reckon you'll need at least:

Cluster gear;
1st gear;
4 x synchros;
Bearings;
Small parts kit.

You may also find the case hardening on the input shaft is gone too. You can bush the cluster gear with a sintered bronze bush which makes rebuilding a breeze.