I honestly don't think just porous material for the gasket will achieve anything like the cooling airflow needed. The current cover provides air forced into cover and exiting in space created by proud mounting points in lieu of washers used previously, plus aperture at bottom of cover which serves as drain as well.
Moto Guzzi provided solution to similarly located alternator rotor. A plastic cover had extended vent/drain at the bottom. This created low pressure area behind, in turn creating airflow inside cover via a spacer between cover and crankcase with a lip in the profile that didn't allow direct water ingress.
Maybe you could create something similar with your CAD profile but would still need to add the cooling slots to an earlier type cover.
