There is one thing that kills a d20 Pendragon game.
Magic.
In the Chaosium game, magic is rare, extremely dangerous, very subtle, and difficult to do.
In d20, wizards lob fireballs, disintegrate people and objects, and alter reality.
In the Chaosium game, faith is a very real attribute and can cause effects such as inspiration in combat and minor miracles.
In d20, clerics, raise the dead.
Magic is so powerful and so pervasive in D&D that it totally upsets the chivalrous fantasy world of Pendragon. A conversion would have to make magic a lot less powerful for it to maintain the flavor of the game.