This photo shows that the right edges are curved and the left edges are square. I have included a yard stick to help provide dimension.
This photo shows the front edge of the screen and its track.
This photo shows the window slid open so more detail is visible.
This photo shows the trailing edge of the screen's track.
This photo was taken from the outside. The screen is held in the frame just like a screen door or window. It has a rubber bead in a groove with the screen underneath the bead.
It would be nice to find a replacement screen but if you strike out I would suggest this. Get a piece of Masonite, quarter inch plywood or something like that. Cut it to fit over the hole. Staple, glue, or otherwise attach screen material to it. Then, using some Velcro, fasten it in place. That's cheap and fairly easy to do and will give you a screen so you can open the window.