Not going into the isolation question, don't need my 2p on that point.However, regarding screening, don't get confused. There is not enough current cursing through the regulator (as is present in the plug wires when the magneto dumps its load once a revolution) to cause the kind of electromagnetic field necessary for electromagnetic interference in radios. Even if there were, the aluminum regulator cap affords no shielding to electromagnetic waves, being non-ferrous. Now then, I believe the spark itself is another issue. I think it is a source of mostly electrostatic energy, which can be shielded by aluminum.
I know. "So, what?" and "Who cares?"
Richard L.