Plastic welding would require a repaint. I would try Permatex Hysol of which there are many formulations. Probably a plastic panel but forget the number for plastic. The stuff can be worked into the crack and then clamped - all this of course with the panel off. A fiberglass panel is easier to fix since you could glass the crack from underneath with milled glass fibers and cyano.
I remember quite some time ago a body shop using some sort of schmutz to repair a plastic Ducati fairing on a customer's bike. The stuff was extremely strong, but I can't remember what it was. It may be worth a few calls so you don't have to experiment on a piece you want to keep.
When I got my ST2, it had all sorts of broken/cracked tabs in the fairing - apparently from parking lot/garage drops. My research led me to Plast-Aid, which worked very well with no failure of any repair over the last 4 years. I haven't dropped it, however. I even "replaced" a missing bit of a lower section by using aluminum foil as a form and filling with the Plast-Aid material. The only issue is that the material shrinks as it cures, so it took a couple of layers to get the thickness I wanted.
I bought the "big" kit, and still have more than half of it.