I think that converting to the Delrin ball on a stainless post would be pretty easy. You could probably keep using that base.

In the meantime you could make a plastic Chip similar to the Hobie 16 the shoved into that mast base to prevent further erosion of the aluminum.

Or, you could cut the top half of that ball off and replace with a half sphere similar to hobie 17 or 18. I have a Hobie 17 mast base you could play with for the price of shipping wanted to try that. if you sent me the dimensions on your mast roughly I could tell you if it was even close to fitting.