Yeah, get it off and apart and go from there. The shell is probably fine.
You can cut a big rectangular window in the side of the rudder with a Skil Saw and open it up. If the SS is OK you can readily rebuild
of the SS weld joints is difficult, but you just have to do the best you can. If you are just sailing locally, it's different than going offshore
I did mine still in the boat, but out would be easier. I cleaned the waterlogged foam out of mine, ground the entire outside clean, reglassed in the big cut out side. Then I filled the whole rudder using two 3/4" holes on the top with lightweight epoxy
filler. Pour in one hole until it comes out of the other.
Of course buying
a brand new rudder would be nice, but I doubt it's cheap
I would guess 50% of the old boats out there have waterlogged rudders but the owners don't know it. Two of my boats have had; one was 6 years old, the other was 7 years old at the time.