Alice,
	Plastic screen is available at the hardware store and the
screen store, You could sew this over the existing screen, whith
a little strip of cloth covering the ragged edge of the screen. Or you
could remove the old screen by removing the seams that hold it in
and then sew it all back together.
	I'd be inclined to take a page from the tent manufacturers,
if this isn't already done, and install one side of a zipper to the screen,
sew the other side of the zipper to the canvas top. Therefore be able to
change screens more easily in the future.

	Bill