I had this problem when I bought the boat.  It's kind of a PITA to fix.  I hired a fiberglasser to cut it out and build up a solid fiberglass square to replace it, with a fore/aft drain hole at the base of it.  Has worked well for about 4 years now, except that he didn't quite build it up high enough, so when I next remove the mast, I'll build it up about another 3/4 inch.  Then it should last forever.

Hi Bob, discovered a saturated but still solid, teak base block under the fiberglass that the cast mast step was lag bolted to. Pulled the mast and while inspecting, found lag bolts to be loose because the area around the threads was mush. I drew crisscrossed lines, tic tac toe style by which to realign bolt locations and cut out a "window" leaving approx. an inch of the glass still above the edges of the block. Drilled, chisselled, etc to remove the block in pieces. dried area with fans, heat lamp and glassed in, new teak block in crisscrossed layers to original thickness, glassed in the fiberglass panel I'd cut out earlier, and made drain holes to the bilge for mast "ingress water". I hope
all went well.

FWIW, G-10, although a little pricey, would be an excellent alternative to build up a block to replace a rotted mast step support.  It is as strong as steel and I have used it where I needed to reinforce an under deck to install crossover "Lemae Winchfeeders" on a J/120, and to create strong points to mount companionway grab handles where no mounting point existed where I could lag the handles into (excuse the grammar.)

kk