G'day MadGadget, and welcome to the ODK Forums.
That nut is probably just rust-bound to the stud. Are the visible stud threads still OK? If so, the following applies.
If you've got the stud out of the crankcase, the existing nut can be split, to get it off the stud. Then it's an easy job to clean up the stud thread.
The way I'd go about it, is to use a thin cutting disc in an angle grinder to cut grooves along two opposite faces of the nut hex, i.e. parallel to the stud.
Then place the nut/stud on a hard, solid surface - e.g. the side of a mash hammer, or an anvil. Set it up with one of the grooves upward, and use a cold chisel and hammer to split the nut.
