Look into the particulars of the new ORC license. The Pathfinder rules are pretty much totally available for you to reference as you like as a 3rd party game. Dark Sun is probably the more thorny part IP law wise. But of course the 5e conversion has been around for ages, so there's even precedent for that.
If I were doing it I'd use the 5e conversion as sort of a template. Like obviously don't copy them exactly, but use them as an example in terms of what parts of the IP they talk about directly and what they don't, and generally how they thread the needle.
