Adjustable stem busted - Do I need to replace the whole stem?
I'd call that part garbage, sorry.  You've overtightened it enough to tear apart the metal, and that toothed flange is probably custom.  

I would suggest replacing the whole unit with a different brand.  The one pictured APPEARS to be a threadless stem, but with the steerer clamp down below and the hinge/knuckle up on top somehow.  This is quite unusual.

Now that you know what kind of angle/length you prefer on your stem, consider purchasing one that fits those specs.  

A conventional non-adjustable stem might look more like this: