When i get old photos that need to be restored and the photo is tattered and torn, with a stained and faded background, perhaps with cracks and tears, it would be very tempting to replace it.
Short answer don’t!
I get two or three emails a day from wannabe restoration artists who replace backgrounds routinely. Frankly I am not a fan of this practice. Most are done very badly, with the old, ‘render clouds’ filter and then over blurred with no attempt to match the grain.
Take time to repair the scratches, and tears, correct the fading and stains and when your done with the initial clean up you may find it hasn’t improved that much. Try experimenting with the dust and scratches filter to even out the tones in the background. Then when you have found a setting that works, add a layer mask and reveal the restored image through the cleaned background. You may need to match in some grain at this final stage. The background should now look much more convincing than if you simply used a filter to produce some random, over smoothed clouds.