Whatever you do, do not buy a "stock" centerboard unless it is significantly cheaper than a CB with a correct foil section.
Suppliers don't always indicate whether the design is "stock" or whether it uses newer molds with good foil section, so ask.
If you choose a repair, you can use the occasion to "improve" the foil shape somewhat. Not as good as building your own or purchasing a well designed one, but definitely cheaper and (in the end result) better than purchasing a "stock" centerboard.
Is a repair feasible?
I would think so. Having water in the CB isn't necessarily a fatal flaw; there are many CB designs that are flooded on purpose (to reduce buoyancy). This does require a drain hole, though, otherwise you will get frost damage.
About the crack. If it is in the leading / trailing edge that would be expected the way the CB was put together from two halves. Adding a layer of laminate to wrap across the crack would probably be a sufficient fix.
If you can pardon a very crude drawing:
As you can see, across the leading edge, the stock profile needs to become blunter to approximate a reasonable foil shape. That gives lots of scope for adding not only filler but laminate. In an ideal foil, the maximum width would be at 1/3 of the chord (more or less) and that would mean that adding laminate in the entire forward half of the CB would not interfere with improving the hydrodynamics a bit.
For the trailing edge, there should be room to add around 1/2" and that can be done with 3M Marine High Strength Filler. It's polyester-based, but reinforced with chopped fibers. It's a "structural filler" that means it's strong enough for applications like this.
If your CB shows rust, then you have rebar, and water has gotten to it. Yes O'Day did use rebar on some DS. Any repair of that would be temporary, because the rebar will continue to rust and expand.
This picture shows the "stock" profile and where you can modify it with a bit of fairing compound.