You might find someone responding here with a used rudder. Failing that, you might be able to purchase a stock rudder from DR marine. (There's also a "DS3" rudder for sale; despite the qualifier, I'm not convinced that it would be impossible to use on or make it work on a '68 DS1)
This link for a complete set, but if he lost only part of the rudder, you might find that as well.
https://drmarine.com/proddetail.asp?prod=DS109Best to place any orders by telephone; also to confirm that the listed item is still available.
These rudders are also not beyond the ability of any semi skilled woodworker to build on their own from scratch (or just the rudder head and rudder blade and purchase a tiller). Digging back a few years in the "Repairs and Improvement" sections are
instructions on how to build head and blade. Takes about two weekends, working part time. (Most of the time is spent watching paint dry). Materials would include plywood, epoxy, a bit of fiberglass cloth, a pair of pintles, a stainless steel carriage bolt, about 1/2" or 5/8" diameter, and a longer one about 1/4" or 3/16", both with wing nuts. Plus assorted nylon bushings. That plus fairing compound and marine paint.
Anyone with a more developed skill set might even realize their own improvement. For example replacing the plywood with a plate laminated from thin strips of wood with alternating grain.
But sometimes, the goal is to just get on the water. And then, either a used one or a stock rudder would serve the purpose.