Can large plate glass window frames be repaired if they are partly corroded?

Steel can be welded but the window must be disassembled first. It would probably be cheaper to get a new window made. Get a quote from a window manufacturer.
To stop the rust, use a rust converter paint. Remove large flakes carefully, do not break the glass. To make it prettier after converting the surface rust you can fill the area with plasti , bond or similar then sand it smooth and paint. Note: it will not be as strong as the frame was originally, but it will be as strong as the rusty frame was