Bonjour mon ami, bienvenue au BKP forum :)
Although I don't own a 513, I have played one & inspected it up close. From my memory of it (& looking at some pictures), you might need to rout some of the body to make a full sized humbucker fit - as well as the little curved bit in between the two coils, the two singles together on a 513 seem a shade thinner than a humbucker.
If you used a mounting ring it would cover up an routing that had taken place - you just couldn't put it back as it was. If I had a £4k guitar that I didn't really like, I'm not sure I'd do it :)
Like I said though, I don't own a 513. It would be best to get a full sized humbucker in your hand & hold it next to the 513 to see if it looks like it would fit...