It shouldn't be too hard to fix, as long as the rest of the fly is in good shape. The first thing to do is to decide whether you want to replace it with a clear panel or fabric. If you want to replace it with a clear panel, you need to figure out what it was and where to find a replacement. Many of the clear panels are Vinyl. Vinyl starts out clear and flexible, but gets yellowed and brittle as it ages. Next, you need to know how thick it was. You might find some clear vinyl at an auto upholsterer and they could probably install it. If you don't want to fool with a clear panel, the best alternative would be a white fabric panel. Your local seamstress isn't likely to have the right material on hand, but you should be able to get some from on of the MYOG fabric suppliers. Removing the old stitching and sewing in a new fabric panel should be something that any one who does alterations would be capable of. You don't have to use white, but white fabric won't color the light coming through it.