Here are the "As Found" pictures of my ODay 20. I don't know what was originally mounted on the transom rail. I also don't know what normally attached to the back side chain plates. The scariest picture is the stripped out interior. I built conversion vans for four years, and this I can do. I was missing a back seat hatch. I made one out of PVC "rot free" board. They sell it at the big box hardware stores. I works like wood, but you use PVC glue. I think I'm addicted....
That boat has nice lines to it. I think that it can be restored and brought back to almost new condition. Looking at the back of the transom, I can see part of the old outboard bracket bolted to the stand-off block on the Starboard side. The rubber is missing from the gunwale moldings. If these moldings are still in good shape, you can buy the rubber for it at D&R Marine. I love the way the combings are set up with the Gennie winches. I'm surprise that there's a steaming light on the mast. There must be an anchor light on top of the mast but I don't see it.
As far as the inside of the cabin is concerned, I would not try to make it look like a stock O'Day 20. Who the heck would know what the inside of the cabin looked like with the exception of someone who owns a boat like yours? I would design the interior of this boat with your own personal wants and needs in mind. Granted, you're not going to have a lot of space in the cabin to build everything you want, to make it a comfortable pocket cruiser, but I think it's important to perform the mods in a boat that will suit your tastes and needs.
I really believe that this boat can be the perfect trailer-sailer. You can park it in your yard and just trailer it to your favorite waters, launch it, and enjoy the same adventures that I do on my boat. The best thing about it is that when you're through, the boat comes home with you. No mooring, club fees or slip slip fees to worry about.
If you decide to go the trailer-sailing route, you should buy hardware for this boat that will make the mast raising/lowering setup time fast and easy, and by that I mean using quick shackles on the main sheet tackle, and a press button quick pin on your fore stay turnbuckle that saves having to mess with rigging pins and cotter pins.
A lever type turnbuckle adjuster on the back stay for throwing slack into the forestay comes in handy for lowering the mast. Whatever you can do to make this boat user friendly, is key.
I'm really not familiar with the O'Day 20 but if you need some modification ideas, check out my Owner's Photo Album on this forum under Trinkka.
Good Luck!
Joe