I literally had this exact repair done on an old 80s vintage 6lb aluminum Worthington. I took my tank to a propane shop in Concord, California. The guy put a new OPD on and it took him about 30 minutes, I was shocked it went so fast. My old OPD was legal but the non replaceable sealing gasket had failed so valve had to be replaced. History here is the OPD that had to be replaced was one that I had had installed years ago to replace a non legal valve like yours, There is no question it is doable, I just had it done! He charged me a whopping $40.00…best deal ever. I wrote about it on the forum somewhere for the record.
He also had a perfectly serviceable aluminum 6lb tank sitting in his “old tanks” collection. I wanted to get it as a backup but my Wife would have killed me ( I have a lot of Freedom parts in my/her garage). I had thought to buy it and offer it to folks on the forum because there are hard/expensive to come by but red tape prevented shipping so I gave up on that idea. Unfortunately It turns out it is very difficult to ship used tanks in the USA, there are lots of certs and red tape involved.
The only issue with the new OPD is the length of the float. it can be too long to negotiate the body of the narrow tank. But there is a pretty simple fix, cut the float down to size. They are a solid foam material.
If I were you I would measure the thread on the tank, verify an OPD size , buy it and give it a shot. If they won’t ship it to France, ship it to me and I will ship it to you in France, My Wife has relatives in Paris and we ship stuff there from time to time. No big deal. PM me if you need help with this.
BTW, that transom is absolutely beautiful!
here’s my post from a few months ago