There were two styles of R safety mechanisms in the James Wayne era-a push button safety, and the more conventional rotating lever. The parts from these are not interchangable, and, of the two, the lever is the more secure and fool proof design. You haven't been clear about the exact problem, but, if the gun is a push button safety, and the button has rotated in the breech block, the safety will not work. It is a simple matter to remove the breech and inspect the retainer spring to see if it is riding on the flat of the safety button.
My conversations with Geoffroy have led me to believe that general repair work gets in the way of his engraving, and he doesn't actively pursue repair or stockwork, a pity, as he is quite qualified in either area.
During the decade + that I imported Darne shotguns, I utilized Kirk Merrington for any warranty or repair issues that came up. At that time I set Kirk up with communication with the builder for any technical matters he might have to deal with
that I couldn't answer. A FAX with a simple line drawing and short explaination from the French makers can be an extremely valuable thing in the event repair becomes needed on a Darne, and I suspect they would still be willing to provide that to Kirk.
The Darne works has been sold, and while I have no clue what the current owners plans are, then, as now, the trouble is spare, rough parts for repair work.
Kirk has a website, to a large degree his wife handles paperwork and email. I would suggest you contact him for repair of a Darne. Google him.

By way of illustration:
The photo shows the safety button spring on an R model Darne, in this case a rotating lever version. The pen is pointing to the end of the spring that must ride on the flat of the shaft of the safety button or lever. You may have to rotate the button to get that to happen. If one side of the safety button is larger than the other, rotate the large side clockwise, as the shaft is two pieces, and rotating counter-clockwise will result in the shaft coming apart. Don't use a vise grips to do that, mate.



Best,
Ted