In the original game the mouse sensitivity is still high even when the slider is set to the lowest value. I haven't found an option in "system.cfg" (located in the main game folder).

Eh?! Not that high at all. Go change the sensitivity in your operating system and your mouse or its drivers. If you use Windows, turn off that mouse accerelation BS first if you did not already. It really makes it unnecessarily hard to play accuracy games.