If it is the thermostat, be sure to replace it with a 87 degree C, and not the 83 degree.
Either the cat converters are forgiving (I'm pretty sure mine is original; Even with slipping cams and 133,000 miles it works well) or the Motronic is clever enough to keep thins in line.I ordered a new thermostat, gasket and bolts from Alfa Parts. The next day I had them on the door step. Drained off enough fluid to pull the thermostat, gently removed the 3 bolts. I was very concerned they were going to break off, fortunately they didn't.
The thermostat was wide open after it cooled off.
Cleaned the gasket, swapped the parts and put in new antifreeze. Now the car is putting out some heat from the heater again. I was mainly concerned that the catalytics were going to over heat from to rich of a mixture, but I think the computer adjusted for that.