You can choose whatever material you want to redo the centerboard
but the boat won't act exactly the same. The lighter the centerboard the more tender
the boat will be. I remember sailing Ray Greene Rebels back in the 70s and they all had steel
You can buy a piece of steel
roughly the same size as the one you have and encase it in fiberglass
cloth and epoxy
resin to make it less prone to rust but it will rust eventually.
If it were me, I'd laminate some fiberglass
mat, roving, cloth together to be the same thickness and use that as the centerboard. Pretty expnsive for materials if you use epoxy
resin. Not so much if you use polyester. No more rust and just a bit lighter!
Another option is stainless steel. That would be expensive. Another option is aluminum
plate and that would corrode too but maybe not as fast if you coated it right with zinc chromate and a good two part paint.
Good luck in whatever you choose.