I just went through this with mine and I was able to get that nut back on by putting a little grease on the bolt so the nut would stick to it long enough for me to get my fingers in there to start it. Good luck!
Ed
Believe it or not Sears sells all the parts for these things.
http://www.searspartsdirect.com/partsdirect/brands-products/Kubota-Parts/Walk-behind-lawn-mower-Parts
I have a shop manual for it if you are interested.