The stud is a specialty bolt not easily replaced. With larger headtube sizes the past 25-30 yrs brakes have switched from
fully through bolts with a hex nut externally accessible to shorter bolts and hollow tubular nuts with allen recesses. These
are available in different lengths to accommodate fork sizes. In the era when I was futzing with old/new brake and fork
issues I did manufacture a replacement bolt for a shimano brake which included a step up from the 4M threading for the
attachment bolt to 6M threading for the brake body. It worked. My experience with Varsity was in the '70s so can't
comment on your specific brake bolt, but looking for a Shimano hollow tubular nut in an appropriate length might
get you better metallurgy so as to avoid the allen recess rounding out.