Had the same problem on my FZ6 but as a temporary repair I just got the new nut to nip the sprocket then had the nut welded to the shaft, thinking that winter I would replace the shaft, that was 4 or 5 years ago and 20,000 miles plus done.
I put a new chain and sprockets on, soon I will have to replace the front sprocket as it's starting to wear, the chain and rear are fine, I know I should change the lot but the front does 4 times the work of the rear, my thinking is that it should see the bike out and maybe me as well?
I'm hoping to get a new bike soon the the milage might drop to a couple of thousand a year on the fz6 as I also have a MT that I do around that milage a year.