Uhh...if you are worried about cracks in repaired fairings, would you not have the same problems with a fairing with a piece grafted on? Is that not a repaired fairing also?
IIRC the fairing is ABS. It can be welded and be stronger than when new.
I didn't say how to adhere the grafted section. And yes, I would plastic weld it in a few spots and then use Legos melted with MEK and window screen to build up the area after removing the paint on the back side. Just rebuilt my dad's fairing this way. You wouldn't know it from the outside after it was repainted. The trick is to make the cuts for the grafts as precise as possible and champher the edges so the new stuff has some grip area.