Devcon or Gorilla two-part (non-yellowing) epoxy is what I use to set large cracks or reattachments. But I then use super glue and insta-set to hold a small portion together so it doesn't shift or move while the epoxy cures, like Vince Vell does in his videos. Some applications may need something more like Aves Apoxie, or some smaller ones may just need some super glue.
Since your item broke off at such a critical weight load point, I'd actually want to put a rod in there to strengthen it too, then epoxy it back together myself with the two-part epoxy, and then touch up the paint. So, pinning it would be the way I'd go as well. I'd do epoxy to put in the pin/rod, then epoxy that to the other piece and hold it in place with mechanical support (like the objects to hold it up, or use super glue in a small section and insta-set to hold it while the epoxy itself sets up.) There are tons of videos on Youtube that'll walk you through it and tell you what products were used, but Vince Vell's are the best.
Example epoxy:
https://www.amazon.com/Gorilla-Epoxy.../dp/B001Z3C3AG
I am still in the novice range for customs and repairs though, so some more experienced folks may be able to speak more to the fact if that won't work for Polystone, but I've not had any trouble with it on any of my pieces yet.
