how to remove discolouration/stain from old black and white photo

As ​@CORCTON34033231 suggested, for such big areas where image information is completely lost rather than simply overcast or saturated, it would be best to use generative fill. You can leave the prompt empty or add a few words describing what is in the filled area (a shirt, a wooden cabinet, etc.)

For stains like that, the Remove tool (or Spot Healing Brush) works best: brush over the reddish areas and Photoshop samples the clean surroundings to rebuild the detail underneath.

Since it's a black-and-white photo, a Black & White adjustment layer will also kill the red color cast instantly; then heal any remaining tonal blotches with the Healing Brush. For larger blooms, try Generative Fill: select the area, leave the prompt empty, and generate.

Work on a duplicated layer so you stay non-destructive. Here's Adobe's guide on the healing tools: https://helpx.adobe.com/photoshop/using/healing-examples.html