If there’s visible cracking below, then I’d suggest that there’s compromised materials right up through the support beam and core, clean through to the mast step. No shoring you do from below will stop the mast from sagging into the deck. The high compression loads exerted on the mast step area dictate that any fix here be strong and first-rate.
There are a few different ways to go about this but I think the simplest and strongest is to laminate a beam under the mast step to spread the loads to un-compromised areas of the deck. You could make another to do the same thing on top of the support pole to completely bridge the week area from below too. Don’t forget to inspect the support for the pole’s base!
I have made a similar fix to this aboard my C22 and while I have already gone into detail in another topic, I could give you more specifics should you think you’d like to explore this option further.