I had a neighbor who would get wrecked bikes and fix the plastics. He would press some wire mesh into the back with a soldering iron or similar before welding the plastic to make the joint stronger. If it's somewhere visible though, this may not be preferable. That's all I've got.

 

I helped a buddy plastic weld the fairings for my wife's Ninja, everything held well until I flexed the fairing "pivot" too much while installing. Even then, only part of the weld split.

 

The mesh idea works best for strenth. I had someone ride my bike & crash it with a new givi e45 on it. Several large pieces of the polyproplyne(sp) were punched right out. With the mesh(screen) method I rebuilt the missing areas & it was structuraly sound to use. I used the iron type welder that heats up & melts the rod as it passes through. You can use a soldering iron to heat & melt the mesh into the plastic. You can also get huge rolls of ABS or PPL welding rods cheap at McMasterCarr. It is good to practice on a test piece before you have at the real thing.

 

I did some plastic welding on my fairings and found it to be a total pain in the ***. It worked and the fix came out strong. But I found it very hard not to burn through, warp the plastic, get bubbles, melt surround areas, adjust the air flow to make it any better, etc. But it turned out much stronger than superglue! And after some sanding/grinding/bondo/sanding/painting, it looks ok on the front side. Still looks nasty on the backside, but nobody's looking there.
I see why fiberglass stuff is better track use where crashing and repairing could be more common. I would much rather work with glass and resin than plastic welding.