Finding the pivot pin should be fairly easy due to the limited places where it can be located with the board down in that position. First, finding a place to stand while you work will be a problem unless the water below the boat is very shallow. Second, locating the pin will require carefully looking at the sides of the stub keel in order to see where the ends of the pin might be indicated by hollows or bulges, or other imperfections in the stub. Perhaps looking down into the stub keel from above will be helpful if there is a way to access that hollow area. Another "perhaps" would be looking up from below along the swing keel to see the pin. You will need to discover how the pin is secured. That will probably require some removal of material covering the ends of the pivot rod.
Is the swing keel made of cast Iron? Finding a replacement will be the largest difficulty. Hopefully others will respond with ideas. Best Wishes.