A nut on my bike is rusted and doesn't accept a bolt beyond first few threads.
What can I do about it short of grinding it off and welding a new one on?
Either use a freeing fluid such as "plus-gas" (btw WD40 is not the same WD stands for Water Displacement...) and gently work the bolt through. I have had success with getting a duplicate bolt and putting a hacksaw cut into the threads to help chase the rust out.
Or get the correct tap and re-tap the threads in the nut.