Flux 2x 3S power
 

Never had this inner part of a diff of my Savage Flux broken like this:

http://gallery.weyland.be/d/8188-1/IMG_0724.JPG

Now, is there an aftermarket part with which I can replace this HPI stock part?

Not that I've seen or heard about. I've not read about bevel gears breaking like that. Those are castings and yours prob had a flaw which lead to the failure.