I bought an aftermarket front guard made to fit a Triumph T100, as it looked like it would work with the Guzzi 18 inch wheel. Sure enough it did, however it had four mounting holes pre drilled, which were a few mm wider than the fork brace I had to mount it too. After much head scratching, I had a 3mm alloy plate rolled to match the curve of the guard, then shaped and drilled it to match the guard nicely. That done, I had a couple of alloy strips welded to its top.. these will have their tops milled flat, then a couple of countersunk bolts can be used from the underside to bolt this bracket to the fork brace. Well, that's the plan anyway. It doesn't look good yet, but once the welds have been cleaned up and the whole lot polished, it should hopefully look ok.
Though the dog wasn't too impressed..