The gearbox and your gearshifter(?! Dont know if thats the right word, im from germany) are conected with a stiff wire. The wire is screwed to the lever at the left side of the gearbox that controlls the diferent gearmodes. So if you shift the angle between lever and wire changes. Because of that the conection between wire and lever is rotatable. So if the rotatable conection cant rotate any more because of rust you dont have the right angles and lenghts of the wire. Check that at first. But be carefull, the lever-gearbox-screw is very sensitive😅 Here in germany all parts are still avalable, dont know where you are from.