I have the same problem with the beam in my bilge. I was planning on shoring it up this winter with a few left over pieces of teak from a past project. Ideally you would need to take the mast down, remove the interior teak trim, remove the compression post and remove the rusted "I" beam. At that point I would replace the beam with a stack of structural fiberglass glassed together leaving room for the shower sump pump hoses. I would also replace the small section of flooring that sits under the compression post - probably with more structural fiberglass. The structural fiberglass can be bought at McMaster Carr in assorted sizes.
When finished your support would last forever and be rustproof. You might also think about the deck mast support at the same time since the mast will be down. I did my deck mast support a few years back and added a mast plate under the mast base. It is perfect for al of my cockpit led lines.
You can see where the old stand up sheaves were mounted around the mast base. A total of 24 holes in the deck that were filled in and painted over. No more leaks. Please excuse the pic of the mast base plate. I had not painted the area yet.
