Believe it or not, there is one listed today on the eBay UK site, which judging by the price is made from some rare earth precious metal. Search Vintage BSA Plunger Twin Front Engine Plate Spacer as I am not cute enough to post a direct link.
The part is basically a steel pressing to give a fancy profile, welded onto a spacer tube. Easy to make something that looks original, either fabricated or carved from solid.
Swarfy.