The trouble with Narnia, Laura, is that the last 2 books, The horse and his boy, and the last battle, have a very well defined set of baddies from Calormen, who have a very Arabic feel about them, (in an old "Sinbad" or "1001 nights' kind of way.) even if their god was called Tash or something.(It's a jong time since I read them.)I would also love to seem someone finally do the rest of the Narnia books as films!!
I really don't know enough about Lewis to say if he was actually against Islam, though he was very clearly pro-Christianity. (see Screwtape, the Space Trilogy etc and his life in general)
But either they'd have to make the Calormenes look different from their book description or risk a lot of backlash. The books have been done successfully on radio.