Your vessel is a Ted Brewer design and Ted passed away a few years ago. He still has a website so it may be worthwhile to attempt contact as someone may respond. The
designer would have had the specs and manufacturer of the extrusion.
As a last resort most any small machine shop can obtain a piece of solid
aluminum which can then be milled to your masts contour to fit both the I.D. and mast step dimensions. The center can be drilled if needed for a wire run.
Locating an identical mast section for a forty year old vessel will be challenging at best. Good luck.