So this stuff burns but does not melt?? Or did I read that wrong? If it melts, it is a themoplastic, if it only burns, it is a thermoset plastic.
And it is structural? Must be because the headlights are mounted on it.
Maybe SMC - a thermoset plastic sheet. I’d test a small spot with polyester resin intended for fiberglass. If it sticks well, fix it with that. If it doesn’t stick, try an epoxy fiberglass resin like used for boats. That stuff sticks to anything! And the fiberglass cloth will reinforce the area and make it way stronger than JB Weld alone