I just did this part of the rebuild of a champion 400 -- it requires a spanner....that I didn't have. The nut is threaded as was mentioned. I seem to remember that the holes are appropriate for a 1/4" round stock (such as old/used drill bits that sell for 25 cents at the hardware store). I took two of them and cut off the 'drill' portion and mounted the 1/4" HSS stock into a piece of bar stock with holes drilled at the appropriate diameter required for the aforementioned nut. By mounting, I mean welded. I also welded a small bolt to the bar stock so I could use an end wrench to get the appropriate torque.
On another note, have you found a source for parts? The worm screw at the bottom of my gearbox is trashed.