A bit of a lengthy video, but here's the reassembly of a Triumph 500 gearbox. Hopefully anyone out there who wants to try this can see it's not hard at all. A little bit of prep goes a long way.
Parts Books: https://partsbooks.britishonly.com/
Repair Manual: https://www.classicbritishspares.com/pages/workshop-service-and-repair-manuals
DISCLAIMER: I am not a professional, I am just a guy who works in his garage on old bikes and tries to do the best he can with what he has. Any opinions given are just my own.