There's really no way to say until you get in there. You can use some Git Rot and that should help. But it all depends on the exensiveness of the rot. BUT... you should know that the rot will likely be (it usually is) about 3 times as much as you are thinking.
If it was me - seeing as how the platform is so accessible and easy to work on... I would remove the fiberglass skin from the underside - maybe a section about 6" bigger than the bolt pattern/size. See what you see and fix. Then epoxy the skin back on and lay a piece of cloth tape over the cut (after bevelling the cut). You can finish it off nicer, if you wish - but doing that will be 100% functional.
Side note... at 20 years old, your boat is 10 years overdue to reseal lot's of things. This is a maintenance thing that should be done. I wouldn't blame the factory for this - well, not completely, anyways - your boat may have been built on a Friday and they may have forgotten the sealant
But, point is, it's up the boat owner to make sure things get re-sealed when needed. And... "needed" is defined as: "Before it's needed"