Look here
Look on this site under the Resources tab and then go to Documentation and Manuals. The only Oday 23 "manual" is downloadable from there for free. As you can see, it is a double-sided 11 x 17 sheet of paper will probably be a disappointment if you are looking for detailed drawings. I also remember seeing some photos on this site of the decks of an Oday 23 posted by someone who had installed blocks and deck organizers to run his halyards to the cockpit. Check the archives. If you are looking for info on the backstay, on my Oday 23 (1979), the tang is offset from the centerline to starboard; a one foot long length of shroud is attached to the tang with a clevis pin. The other end of this short length of shroud is attached to a triangular plate (2x2x2), that has holes in each corner. The plate is mounted so that it faces fore and aft, with the backstay affixed (by turnbuckle) to the top hole, the short shroud to the bottom hole. The third hole "points" inboard; the mainsheet is attached to this hole. This arrangement has its good points and bad. On the plus side, it keeps the mainsheet out of the way and the cockpit clear. On the downside, it means the boat has no traveler and that the backstay is subject to high stress.Your other issue about adding a furler is more straightforward: there is no need for any specialized new deck hardware to add a roller furler. It can mount directly to the existing bow tang. One caveat: make sure the size furler you buy fits the tang. My tang needed to be ground down a bit to allow the furler to be mounted. Good luck