I would say just cut a new thread. That would be the most cost effective and least time consuming way as you say. If the current way works and you were worried of the small amount of thread letting go you could drill through the shaft and lock the nut on with a cotterpin. Rethreading would be easier than that though I would say.

Out of curiosity is that mower similar to this one? I tried googling your model and nothing came up.

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=48386#Post48386