The compressor for a 3.0 Alfa 166 is a bolt on replacement if you need to get a replacement. Pattern parts are fairly readily available. The dryer filter is a standard Bosch part for an Alfa 155.
It's a pig of a job to do though. One of those things where it will only just fit through the available space. The oil leak is most likely from the compressor oil filler plug. Compressor holds 135ml of PAG oil from memory.
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If anyone does remove your AC pump you MUST block up the pipe couplings as if left dry, its only a matter of days till the unit seizes and it rendered completely useless.
If you can get some oil in there, all the better, you can buy PAG 50 from Eurocarparts for about £10 for half a litre.
Word of caution. My AC compressor disintegrated at the clutch area causing it to seize and stopped the aux belt turning.