What sort of window screen frame am I looking at?

There are 100 or so different replacement window companies multiplied by customer window sizes. This is no small task. You can make up a screen with aluminum tubing and corners to the size of the window , or just get replacement windows to install yourself or a company. Chances are new windows would be a major upgrade for the home. I think my bottom line here is that unless these are Marvin or some other major brand you’re going to be frustrated. These are just storm windows , so replacements are around 200 and the screw on to molding. But like I said unless this house is “historical” I’d be getting replacement windows , even one at a time if the budget is tight , just move forward.