I rebuilt the rudder for my Niagara 35 one winter. Not too hard a job but takes quite a bit of time. In my case (and many others) the rudder shaft was stainless and the web inside the rudder was mild steel (I guess they thought water would never get in - but there were wrong).
1. Remove rudder
2. Split the rudder along its edges - cut fibreglass shell with a small handsaw
3. Split interior foam and separate the two halves of the rudder (post and web still with one half)
4. Free metal bits from rudder - get the web replaced with stainless
5. Remove as much of loose material from inside rudder shell as will come out easily.
6. Replace shaft and web (enough of the original left so it fit into the spot originally there
7. Put two halves together with Spanish windlasses and clamps and then put heavy tape over the edges
8. Drill a couple of holes to allow access to voids and fill with slightly thickened epoxy
(not too much at a time since it will get very hot)
9. Remove tape and clamps and cover edges with fg tape and epoxy
10. Fair and paint
I kept the boat for about five years after this repair
and after five years the rudder still looked better than new