I replaced the mast step on my boat (Celestial 48) a few months ago. I have a keel stepped mast and the support under the step was 1/8" glass over wood. Water had run down the mast and over time had gotten to the wood causing it to soften and compress. I removed the mast and removed/ground out the old step support. (A messy job but I needed to save $) Then I had a professional build a form and pour a base of fiber reinforced epoxy. This took several pours as the volume about a cubic foot. (Hence the pro) On top of this went a 1" plate of G10 to which the mast step itself was affixed.
Total cost about $1000 for materials and labor. This was probably overkill but it''s a data point for you.