FWIW, I've a set of MC16 fairings which are pretty brittle due to age. Last crack I had to fix I used a combination of rewelding with a hot soldering iron and then re-enforcement by taking some metal mesh flyscreen and pressing into the plastic with the hot soldering gun. Hopefully it will work The screen was applied to the backside of the panel btw.

Oh yeah, the crack was an old one which had been previously repaired using one of the chemical solvents. Solvent just made the plastic more brittle and wasnt done that well.