I'd check the sails over first. The biggest reason is that there has never been any boat called "O'DAY 21", O'day built a 20 (with 2 different rigs over it's 6 production year run), a 22 that had 2 different rigs over it's 11 production year run, and the 222. There were 2 different 23s with slightly different rigs, plus the older 23' Tempest.
I would think that the jib from any of the above would fit a 1973 23, but I doubt that the mainsail would be a good fit, most likely it will be too long on the luff if these sails came from the fractional rig 20 or 22 (or 222), and too short if from the masthead rig 20 or 22.
I'm not even sure that the sails from a 1977-83 O23 would fit the earlier 23.