Who made the spar? Many yachts of that era were masted by La Fiel (sp?) or Sparcraft. I believe that both of those firms are still in business and could perhaps help source the spreader cap. Or, on second reading of your post, is it just the cap that is missing, or is it the whole tip?
If the shroud(s) are still stable on the end of the spreader, climbing the mast
should be ok . If worried, take a halyard
to the widest part of the beam as a back up.