You could have the ID of the First Motion Shaft and the OD of the Main Shaft machined, sleeved with hardened sleeves (only the sleeve for the Main Shaft needs to be hard) then machined back to original dimensions. I can measure the parts and supply sizes if you go that way.
Denis Welch Motor Sports make replacement Main Shafts.
You don’t remove the Rear Bearing circlip to remove the Bearing and at the same time, dismount the Main Shaft. I believe it states to remove it in the Haynes Manual that your using, but that is for the Moss Box. As I stated in an earlier Post, that book refers more to the Moss Box; the procedure is quite different with the later Gearbox. Get yourself a manual that discusses the later Gearbox specifically.