Home made keels
If finding one is impossible, you may have to fabricate one from steel sheet. Ever notice those steel trench plates at construction sites? That may be your only option available. In fact, those steel trench plates may be more resistant to rusting and pitting that that original iron keel. If that's the direction you have to go, find someone who is very experienced with a cutting tourch, and expect to do much grinding. It won't be cheap. Good luck.
