You need to remove the kickstart ratchet assembly, and the small retaining screw adjacent to the top right casing stud. Then replace the shaft nut to protect the thread, GENTLY tap the end of the mainshaft, while pulling the inner cover towards you. This should enable the inner cover to slide down the 4 long studs, as the mainshaft goes through the bearing inner race. The selector shaft should also slide out of the cover, but do not smack this...it will break a piece off the gear case where the tiny grubscrew lurks, behind the chain sprocket.
With that grubscrew removed, you can take out the selector shaft and strip out the box in a matter of minutes. Plenty on the forum about putting it all back!!!
Easy fix for shaft leaks is to machine a groove for an O ring. Not the best fix, but cheaper than new shafts and bushes.
Swarfy.
Additional. If the clutch and shaft adaptor are not fitted to the mainshaft, the kickstart ratchet assembly can remain in place. The mainshaft will slide out complete, with the cover. Those long studs are parallel when new, over time they often get flexed slightly, so the cover may bind a little as it slides off.
Thanks to Roger, a picture says more than words.