Alf,
Well...some good options have been offered here, but here are a few considerations I'd contribute...
Spraying a clear coat over the signature would be the best fix, IMO, but there are problems with this.
Was / is the tank waxed? Even if not, properly painting a shiny surface requires prepping the paint by sanding and using a wax and grease remover (prep solvent) first. Of course you can't sand over the signature, so the next best thing is to sand up close to it. The next worry is that the prep solvent isn't strong enough to remove the Sharpie ink. I'd mark something with a Sharpie and test this first.
Next you'd need to worry about the clear coat lifting the signature- again, something you can test for first.
Anytime a decal is cleared over, such as the Ducati lettering on your stock tank, adhesion on that smooth surface (in this case the smooth vinyl logo) is less than ideal, and is subject to the clear peeling. The smaller the area, the less likely. For this reason, ideally you would have sanded that area of the tank with 400 grit paper before Mr. Smart signed it...

Yeah, like that was gonna happen...
Clear vinyl was mentioned above, and it could be cut to be slightly larger than the signature. However, as was also mentioned, it will over time yellow a bit, even the best 3M clear vinyl will do this.
So, we're back to painting over it. If you're super careful, I think this is what I'd do...