Dave, I second the suggestion to call Tom Haberman/Aquarius. The ARC17 they produce is a slightly modified version of the SC17, which has the same mast base as the SC15. It is very likely that Aquarius will have a replacement mast base in stock.

Considering the boat's age, it is probably a good idea to remove all the fasteners on the mast base, even if only to check for possible corrosion spots.

About a possible repair, the mast base is cast aluminum and I believe it is possible to weld it, but not exactly easy. There isn't much space for excess added metal around the neck, so most excess weld will have to be removed in a mill. This adds to the cost and will probably result in a relatively weak base, when compared to the original. Still, it is the same part used in the SC17, so there is plenty of built in safety margin. The decision between buying a new part and fixing the existing one will depend on price, availability, welder skill, etc.