Now luckily the Italian motorcycle industry uses a lot of standardized kit. One of which are fuelt petcocks. No matter if Guzzi, Ducati, Cagiva or Benelli - they all use the same M16x1 thread and only have different orientations of the outlet and shape of the handle. And the second great thing: They're cheap - genuine parts can be had for around a tenner and good copies for about half of that price.
Still, get some 16mm aluminium bar and a M16x1 cutting die and cut the necessary thread onto the bar. Centre-drilling (admittedly) is a lot easier on a lathe, but not impossible even with handtools. Only downside: The filter measures 10.x mm, so in order to NOT make your life very miserable, you would want to drill at least a 10.5mm hole...
I simply measured the bolt spacing on an XS fuel tank (47mm) and the base measurements of the backplate (23 x 69mm), corners rounded off to an 11.5mm radius and had the backplate waterjet-cut from 8mm aluminum plate. Unfortunately there was a little misshap and the centrehole didn't come out as a press fit, but 3/10ths oversized.
As you can see in this last pic, I used a high-quality epoxy, which I heated up to around 60-70 degrees Celsius to make it fill the gaps better and left to cure for 48 hrs.