First, as others have already mentions, your boat does not have a "swing-keel", she has a centerboard housed inside the keel. O'Day never built a swing keel boat and made a lot of claims about how much better their Keel/Centerboard designs were than swing-keel boats (of course they did have to admit that the K/CB designs were more expensive to build!). This is not meant as critisism of your terminology, but more as a quick lesson to O'DAY K/CB owners in general. (read attached scan from a 1979 O'Day brochure).
Anyway, back to your question..... below I've posted a sketch of the O'DAY 25/26 centerboard in case you need to make a new one. I too have my doubts that the whole cb has disappeared, it may be hanging by a thread..... but....