It's self-aligning.
The mirror tarnish is not removable in most cases. A new mirror is the answer.
You can remove the screen and wash it (assuming it is ground glass).
There are variations on the hood system. If this is a Standard k2 with side screws, the screen might just sit in the opening, not be attached to the hood itself.
And there are later Rolleis with plastic screens. And some with plastic screen on top of glass.
But in general, the mirror and screens self-register.