Sounds like you have already talked with Ward than.I have contacted Beneteau.
Their customer service in my opinion has always been very helpful.
If you have contacted them and they say the new cap will not fit the old fill, than I believe it should not be too difficult to replace the whole thing.
As long as you have access to the back side of the fill where the hose connects, it should be a fairly easy replacement.
We went through this just last year with the pump out deck plate. Somebody, and I won't mention who, cross threaded the cap onto the fitting and damaged all the threads. I had to replace the complete deck plate and cap. It was not difficult, but I had easy access from inside.
I would think you should be able to tackle it, than of course, I don't know all of the details and its a hell of a lot easier doing this sitting where I am than where you are.