Rig-Rite is the way to go
Rig-Rite, Inc. will be able to help you, however I hope you are very good with a ruler. If you go to their website, follow the instructions to Kenyon Spars, then to masts. You will see descriptions and dimensions and some common usage. If you call them they will ask you to go to the website and figure out for yourself what part number you need. You will need to take exact measurments of your old mast at various points. The problem is that just telling them it was a 79 O'Day 23 will not be of any help, as O'Day put different masts and booms on the same year and model. When I first went to the site for a part for a boom it was very difficult to tell the booms apart, however if you get exact measurments it will make finding the correct part number a breeze. Again, the only complaint was the lack of customer service, however I now can understand why. They made so many spars for so many boats years ago, and they do not want to be responsible for shiping out the wrong part, especially a mast. For example, when I purchased my 79, O'Day 23 there was an identical 79, O'Day 23 in the yard. However, the rigging was not the same, both were Kenyon but different models. Good luck. http://www.rigrite.com