Well there are a few ways to do this.
1- JB weld for plastic wood and abs, just fill in the broken area, let it cure, drill out the hole size you need and tighten it back in there.
2 - fiberglass it - same as above - just a lot stronger, but you should know how to work fiberglass if trying this method.
3 - Using a longer bolt and nut, with several large washers.
You can "pinch" the remaining material with the oversized washers, and you would never know it's broken.
