When I repaired mine I drilled out the plastic rivets then used short screws to reattach the new bracket, worked fine 
Same.
Fibreglass wont work, Tigerseal is pants as above.
In the past ive also used pattern part bumper brackets that screw into the bumper.
Pattern brackets on genuine bumpers...
The top screw holes line up but the bottom two don't. So just the top two holes are holding the bracket on. Still works but in the end i made some extra brackets.
Local parts supplies have this bonding cement? Bonds metals and all sorts. Worth checking (forgot the name but they have demos on the counter)
