To remove the inlet manifold is a big job, expect at least 6 hours to remove and replace if it is your first time doing it.
Main points are that the fuel pump and one of its mounting studs has to come off for the manifold to clear it. This can be done without removing the cam belt as the fuel pump pulley can be bolted to the back plate and so keeping the tension on the belt.
You need to drain some coolant as there is a feed between the manifold and head so if you dont the coolant will run into number 1 cylinder.
When you remove the fuel pump be carefull not to drop the woodruff key and when refitting it use lock tight and dont over torque the big nut, tighten to 50Nm or the pulley can crack
If your belt is due you would be better off doing it all at the same time, Alfa have stretched the change intervals to 90k but with the water pump problems you would be wise to do it at no more than 60k.
I wouldnt recommend trying to clean the manifold, I would just replace it with a new one as they have a better swirl valve bearing set up and its a job you only want to do once, get the latest design manifold part number 55210201 they are about £170 on ebay