)Hi Dean.
Something you could try that we did several years ago in a very similar situation.
Get a longish piece of round bar about a max of 1.5mm (1/16") greater diameter than the ID of your stock. I think I used a length of EN16.
Have long taper machined onto one end from approx. your ID minus about 2mm with approx. 75mm length.
Now take a Oxy/Act torch and heat the first 5 or 6mm red hot, insert your taper and hammer it into the heated section. Heat the next 5or 6 mm and repeat, continue until you have got the full OD of your bar in to the length of your thread. With a great deal of care you can 'stretch' the OD of you head stock enough to rethread it. You may get away with just a 1mm oversize bar. I suppose you would have to judge that from the state of your existing thread.
I know this will 'thin' the wall but not I think to a significant or dangerous extent.
As its already cream crackered I don't guess you have much to lose by trying it. Perhaps you would care to let us know if you give it a go.
Mike