Replace clutch or complete gearbox

The clutch of my 20-year-old Omegas seems to give up the spirit soon. Well, I’m not quite sure because especially at temperatures of -6° the gears are very difficult to insert. Even in neutral position, the formwork lever is difficult to move. That points to the gearbox. Now I’m not sure if only the clutch or the whole gearbox should be changed. With my 2.4i I already had the clutch changed. could you change the gearbox and the clutch yourself if you don’t have any experience with it yet? Does the gearbox of a 91er fit in my 88er c20ne?