Tiller-steered?
What happened to the original? I replaced my rudder this year and was not eager to try to replace that part. I had the new rudder drilled to fit the old (quite imperfect) rudder head.
You might try Bristol Bronze, although the rudder heads I saw in their online catalog were for rudders with posts of smaller diameters than my 32's.
I also moved up a size in throughbolts (drilling the hole in the rudder head out) and rebuilt the shim plates on both sides of the tiller casting. If those are your issues, they're fixable -- and I have a ton (well, maybe a pound) of leftover bronze shim plate stock if you want some. I believe it's 1/16" in thickness.