I'm not sure if he has any parts for the older boats, but it might be worth checking with Rudy at D&R Marine (508) 644-3001. He was the Parts Dept. Manager for O'DAY from 1973-89 and now has his own store selling (what else?) parts for O'DAYs. If the 1969-early 1973 model MARINER used the same (or similar) setup as the later 1973-78 MARINER, Rudy will know how to fix it. The other option is to contact Stuart Marine in Maine, present builder of the MARINER and RHODES 19, they do restoration work on older model year Mariners and Rhodes 19s and may have a source for parts like the hatch guides.
stuart@midcoast.com (207) 594-5515 (from a 2005 parts list, you may wish to GOOGLE Stuart Marine to verify phone# and/or e-mail.