Happy New Year! My dad had an O'Day Mariner when I was growing up and we sailed the heck out of her -- great boat. I know for a fact that it's still being sailed, just not by us. My memory banks are reminding me that it was a centerboard boat, not a swing keel. And because of that, unless you can find a scrapped one somewhere, I think that a fabrication will be the way to go. I do remember dropping the CB for service in the off-season, and it was easily manageable by the two of us. I have a boat with a swing keel, and at 625 lbs, it's a mother and can be challenging and dangerous to service.
I applaud the fact that this old, really well-built boat is still to be in service and teaching young'uns about sailing and life! Good luck and let us know what you end up doing.
Cheers, Ron