That does very much look like a National Cycle shield from this angle. From the 2 mounting spots on the front itself, I'm guessing it has the 2 arms on the back that attach to adjustable fittings to the handlebars? If so, that would be one of their universal fit shield, probably either the Plexishield(?) 2, or 3, depending on length. Do a search on Ebay for the National shields, there are lots of pics of the differing styles, as well as prices.
FWIW, Givi also makes several shapes and sizes of universal shields. I added one to the Guzzi, and a smaller one came on the R65 I recently acquired.