I did not do this so take this with that in mind. I had the same problem on mine. I ended up getting a 3-window canvas and replacing the entire thing.
Prior to that decision, I got some plastic screen, the type you'd put in home window screen frames (with a plastic bead to hold it in the aluminum frame). My plan, and there is info in the archives about this, was to sew the "new" screen, by hands, onto the inside of the canvas, with the top popped, using 2 people to more or less pass the needle back through the canvas, all the way around. If I were going to do this now, I would carefully measure the size of the screen I wanted, take the screen to a tent-making shop here in Salt Lake, and have them sew the screen to a canvas border so I could sew the canvas to the pop top canvas, which would end up and much cleaner job. My local tent-guy is:
http://www.kirkhams.com/
That would still be a 2 peson job, but the screen could be sewn to a canvas with a zipper, and that "module" sewn to the pop top.
Hope that helps with some ideas.

--
Bob Stevens
http://groups.msn.com/BobsPhotoShare/87westysyncro.msnw

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> The pop-up window screen and zippered flap over the screen is broken and
> worn on my '84 Westfalia. I know I could replace the whole canvas, but the
> canvas is in very good shape. I simply want to purchase a replacement
> screen and canvas flap to sew into the opening where the existing
> screen/flap is. Any suggestions where I might purchase a replacement
> screen/flap/window?