you dont need to change the gearbox or clutch, they are the same. you need the manual selector, linkages, and then you need to romove the selespeed actuators and pump from the gearbox so you can attach the manual linkages to it. then you need to fit the clutch pedal and its cylinder and plumb it into the existing hydraulics at the point where you remove the selespeed clutch actuator.
remember the selespeed is just the manual with electric gear selectors and clutch control, using hydraulic actuators.
you dont need to replace the rev countner, but it would be a good idea to remove the unused second digital display.
you can leave the rest of the selespeed circuits, such as the door sensor and its wiring, and the selespeed controller ecu and its wiring.
its cheaper and easier to repair the selespeed car, then sell it and buy a manual. you'll be able to repair the selespeed if you go to alfaowner.com and read the threads, there are millions of them but a few of them contain all the info you need.