I would be surprised if any cover lists Guzzi. You need to look at what a cover says it's for and see what that bike looks like. It also depends on the cover brand. A L from one might be a M or XL in another.
My Eldo with bags and shield will take a XL or 2XL depending on brand. I like the cover to go down to the rims so I would rather be loose than tight. I would GUESS a L or to be safe and a bit baggy, XL for you.
And as always, when in doubt, contact the Mfgr.
Tom