Why is it impossible to make output resolutions of 1920×1200, 1920×1080, 1600×1200 and so on for mode “Line 2x -> Generic 4:3“? It is very important.
The OSSC is not a scaler, it is a line multiplier ( = scan converter). If you want to understand more about how it works, you can read around on this forum and the wiki.
For your question, while you don’t provide a lot of information (e.g. it would be good to know the input line count & hor/vert frequency, which is displayed in the info window), one thing you could try is use Lx4, change V.Active to 300, reduce V.backporch until picture is centered. The output would be 1280×1200. It’s not a standard output resolution, so it might not display on your monitor anyway.