You might try using and old shotgun bore brush that will fit tightly inside te die. Chuck it in a drill, turn the drill on and see what affect it has on the brass. It might remove it and it might not. if the die has deep scratches the brass is embeded in the scratches and prolly can not be removed.
I could say something here about brasso, but I'd probably get blowtorched.
If you can remove the brass, use your eye loupe again and if the scratches aren't to deep you might be able to polish the scratches out. Use a tight bore mop with a liberal, damn I hate that word, amount of lapping compound embeded in it and use the ol' drill again.
Now if this don't work you can figure X amount of labor time plus supplies and you come up with the, darn, I could have bought a set of dies for that.
But you had some fun and learned something.
Forgot to add, those dies are harder than woodpecker lips, so dont baby them.
Best wishes, Bill