[motto]

I think that Terry is right and that cutting off then boring out the remainder of the stub would be the safest option.
Is the casting hollow or solid? Is the other end of the axle discernible? If so does it have a flange to secure it or is it welded or pinned? Maybe have the item blasted to get a better look at it.
What I am getting at is that if the axle was inserted from the other side and is a press fit retained by a flange it may be a quick and easy press out removal perhaps with a bit of heat.
If it is welded in place then boring out would have to be the best option.
If you have the opposite assembly you should be able to identify the thread from that. Other than that I would simply go with a common thread (UNF/UNC) for an easily obtained nut. Who's to know? It will probably never be seen again until the end of time let alone be a problem for anyone.
You may find that a commercially available trailer stub axle can be made to do the job.

Dave