I don't know source on the housing plug. Perhaps another DIYer with access to SP parts will read this , or check with a local SP dealer in your area?
I have noticed a new rubber plug has to be pushed into place and then expands a little to hold in place, but the rubber on older plugs becomes hard as it ages and so does not expand to hold in place very well after being re installed. On a personal R190, I used a tiny dab of adhesive to keep an old plug from falling out. In my limited experience with the plug, it seems like it should be a replacement part included in overhaul kits since it looses functionality from age.
As to using the regulator without the plug, both aspirator holes open. I have not tried, but suppose the different airflow pattern could be noticeable, especially if at depth and breathing heavily from the regulator. As basic as the plug is, it could be homemade from a whittled down small piece of rubber and a dab of adhesive. Or perhaps even a substitute could be found by combing the bins of doodads at the hardware store. Not recommending any of that, but if the regulator was mine and I could not find an original replacement plug...