Two years ago I was faced with having to replace the compression post in my 1985 H-31. The original oak post was rotted at the bottom. Apparently, water infiltration beneath the mast step is a common problem with the mid-1980s boats. Presumably, either the mast step screws or the opening for wiring developed a very small leak which over time rotted the oak post. Fortunately there was no damage to the cabintop fiberglass.
I was able to have a local (a real craftsman) woodworking shop make me a replacement 4" x 4" oak compression post with a channel routered on one side for the wiring that runs through the cabintop from the mast and down to beneath the sole. I also had him make the teak cover trim for the 3 exposed sides of the compression post.
I then tapped in (temporary) 2x4 material on both sides of the compression post to support the cabintop rib, and then removed the original compression post. Measured CAREFULLY the height needed for the new post, trimmed to size, secured, removed the temporary 2x4 supports, and then installed the replacement cover trim.