Tom.. To add to previous comments, the nut will screw back onto the crank just enough to allow the pin to be jiggled out or cut away. On re-assembly the nut has to be re-tightened to some 65-75 Ft Lbs, a quite fearsome figure and a proper peg spanner is essential either a bought item or a homespun solution like KiWi. The usual backyard method was a hammer and drift, which accounts for the damage evident on your nut. Incorrectly tightened or no tab washer means the nut comes undone, the split pin has done its job and stopped the nut loosening completely. Aftermarket nuts are available with an improved design as mentioned by BillyB.
Plenty of pictures on the net of primary covers damaged by the nut, unrestrained by a missing pin, machining away the inner cover.
The crank will need to be locked, either with a clutch locking plate, or reassemble the clutch, put in gear and hold still with the brake. With the barrel off, the crank could be blocked against the crankcase, but this is an unnecessary risk bearing in mind the strength of old alloy castings.
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Swarfy.