I'm not sure Rudy (at D&R Marine) supplies coamings. He does supply nearly every other original part.
The way the woodwork is done on the DS1 is that the deck has an L edge, inside which there's a piece of wood very loosely glassed in. Following wooden boat construction terminology, these would be called "carlins". The coamings screw into these.
Now, the issue with the carlins is that the wood is unprotected, even if not directly exposed to sun / water all the time. It's typical for them to develop a bit of dry rot. Some people have replaced them, some have tried to at least locally reinforce them with penetrating epoxy (SystemThree RotFix, for example) so that at least the screw holes are able to hold the screws for the coamings. (After applying some borax solution to delay further dry rot).
I'm in the latter camp; mine are still hanging on by a thread, so that wholesale replacement didn't appeal (the structural loads can effectively be handled by the coamings, as long as the carlins act as "backing plates" for the screws).
After refinishing your coamings, you might mount them with a bit of closed-cell foam between deck & coaming. That will keep water splashing on deck out of the cockpit, but also away from a narrow gap in which it could be trapped.
Where you have the split L edge, I you could glue a strong piece of wood from the inside (with epoxy) if there's room. (Instead of the hollow PVC pipe I had suggested). That would give you something to screw the missing aft coaming into. (But seal the wood on all 6 sides with epoxy and seal the insides of all screw holes).
PS: forgot to add to my previous post that almost all suggestions to use 3M 5200 should be disregarded on principle. There's usually a better way