Does anyone have a source for the half nut on the kickstart end of the mainshaft? 76-0271 (or 57-2686). It's a 1/2" 20tpi nut, but may be ADM thread, or maybe CEI. I'm fairly sure the nut that was on it was UNF, but it is largely stripped.
The threads on the shaft look ok, but curiously the nut from the clutch end of the mainshaft (also 1/2" 20tpi) did not want to go on the opposite end. Seems odd that BSA may have used a different 1/2" 20tpi thread form at opposite ends of the same shaft. But the first thing I want to do is gather together some nuts of the possible thread forms used.
That nut should be 1/2"x 20 and was used right through to the OIF B50's. Whenever I need a BSA part I type BSA then whatever part number I need into the Google search bar. You normally then have displayed choice from a number of vendors. In your case I typed BSA 76-0271.. HTH
e.g. https://pesltd.uk/?product=57-2686
You've fallen into a bear trap that catches a lot of people. In 1/2" CEI has two threads, 1/2 x 26 TPI and 1/2 by 20 tpi. BSA used the CEI 1/2x20 on a number of engine parts (ie A10 camshafts). It measures very similar to 1/2 UNF so people measure 1/2" and 20 TPI and that comes up as 1/2 UNF in google. A 1/2" UNF nut will go (with force) but the thread forms are different. 1/2 CEI nuts are about but have to be ordered.
What may have happened is someone ran a 1/2 UNF die on one of the threads to clean it up thinking it was UNF, if so then just fit a UNF nut on that one.