Vacuum advance unit can be changed on their own, and seems to fail on older Unos.
From memory, you take off the distributer cap and rotor arm, and you should just be able to see the plastic arm from the vacuum unit beneath the dizzy top plate. It simply clips onto the underside of the top plate, and some gentle pushing on the arm should dislodge it. Then you simply remove one or two screws holding the vacuum unit to the dizzy body, remove it and replace with a new one.
Both my Uno 45's had failed units that caused stalling and poor tickover, and were replaced with new as all the ones I checked in the breakers yards had failed too!
Oh yes, a simple test to see if the vacuum unit has failed is to remove the vacuum pipe from the carburetor (leaving it attached to the vacuum advance diaphragm), and suck on the tube. It should be possible to suck and then let the pipe stick to your tongue with the vacuum!
If no vacuum is felt, like you keep sucking and feel no resistance, then the unit is probably faulty.