I've never handled an 1100 side cover, but in my experience I suspect that they are made of the same stuff as the 1100 belly pan which is polyurethane.
You can confirm this by pulling back the insulation and looking for a molded data tag that has information about the part and should have the letters "PUR" indicating the material is Polyurethene.
3M 5200, which is a polyurethane adhesive, might work, but it will need help. Instead of using thread I would use fine 0.010" stainless steel seizing wire. I would also look at adding a couple reinforcing rods through the joint. I would think that something about 0.020" would work.
About 5200, in my experience it is more of a gap filling adhesive. If the parts have a clean break, the strongest bond may be through the use of CA glue(Super Glue). Regardless of what I used, it would only be to hold the parts together while I made the real repair with wire reinforcements and stitching.
For more ideas, you could look for some videos related to "Bumper Facia Repair".
