Putting a bolt through an enlarged hole was a standard way to deal with the problem in the old days Bob, but in the old days, shafts were big and hubs were massive chunks of cast iron or forged steel. As you said, it depends on how much metal you can safely remove, especially from the shaft. If the design engineer had been any good (which I doubt) the answer would have been "none". Hence the suggestion of putting in two roll pins of the original size, at opposite ends of the hub. That shouldn't make the shaft any weaker than it is now, while making the torsion joint twice as strong as when it was new.
