I've actually created a WFRP retroclone using 5e base mechanics;
Raven's Beak. It takes the base resolution mechanics from 5e and some version of the careers, wounds, and a few others from WFRP. Plus the funnel from DCC, something based on the Chaos Gods, and a few other things from elsewhere.
"Changes are extensive, with the class system and magic system entirely replaced, and the skill and damage systems both overhauled"
This is the crucial point. (Talking to the thread in general now)
While I completely agree such major change is needed, for the context of this thread, the question that needs to be asked: is it even 5E at that point?
I genuinely mean no slight to Neon's project, but for me (and presumably Sword) the draw of 5E isn't the basic concept of rolling a d20 (instead of say 3d6 or d100), it is the classes and spells and monsters etc
Since all that needs to go to "properly" experience the Old World, why not jump to another game entirely? (That's a rhetorical question. I do fully understand rebuilding WFRP on top of the d20 engine can be a fun design challenge, and I really hope and wish you had the best of fun writing it Neon, but I think it is best seen as a curiosity with limited utility.) Everything in D&D (hit points, armor class, saves, a very limited skill system...) is geared for streamlined fun monster combat and once that is no longer the point or one of the main points of the game, I think all of D&D's advantages fall away.
You could totally use D&D for Warhammer Fantasy Battle heroic combat between larger-than-life army heroes, though! (Of course that has nothing to do with the Raven's Beak project. I'm just trying to be constructive by painting a picture of a D&D conversion that actually uses the strengths of the system!)
But nice link anyway. I'll read it more closely for ideas and inspiration!
Cheers