Restoring a 150 yr old architectural drawing?

[Benjamin-The-Historian]

I need some advice on what methods would work best to restore a front-elevation drawing done by the architect of a home that I serve as a conservative society member for. I have this scan to work with. 300 ppi, tif. The biggest problem is the dark staining from water damage. The image also has waves from the scanning process. I would like to make it "square" again, but I don't know if that's possible. I also would like to preserve the original color and texture of the paper, just eliminate the staining, and fill in the small areas missing. I have tried selection masks with hue/saturation, etc adjustments, but the stain is so dark, my method also started taking away the ink lines themselves. I am open to all suggestions. Even if I need to smooth the texture overall and lose the grain of the paper, that would be fine if I could just get a legible, highly accurate depiction of the architect's drawing. 

https://drive.google.com/file/d/1qf7yw-vJFYo6clhPGpm4P3TIolUkXdMA/view?usp=drivesdk 

[Chuck Uebele]

Those are some bad stains, and to restore the drawing will require a lot of manual retouching. You can use frequency separation to get some of the line detail separated from the stains, but since the stains are so dark, doing so will capture more of the paper texture in those areas. You can then mask in the lines - very time consuming. I did a few on the left window in the screen shot to show the difference. I made several frequency separation layers and added curves with clipping masks to bring out the lines in the stained areas and in the light areas for the paper texture. I added a solid color fill layer for the paper color.

[Benjamin-The-Historian]

Wow!!! You got much further than I have. I will research the methods you used, as I am not too familiar with the application of these these tools. It's been about a decade since I last used photoshop heavily, and back then it was not restoration work like this, that I was using it for. I really like the smoother look of the last detail shot you posted. I will view this from my computer and zoom in on your screen shots to see the workspace better, as I am viewing from my phone presently. Thank you very much for pointing me in the right direction!

[Benjamin-The-Historian]

Also just remembered to note that I'm using CC 2019. Currently looking up tutorials for frequency separation. 

[Chuck Uebele]

I used the median filter rather than gaussian blur to make the separations. And I made several texture layers at different setting with the median filter. The first was around 30px. Others were maybe 10px. Trying to isolate the lines in the stained and unstained parts. 

[Benjamin-The-Historian]

Hi Chuck, others,

I have been working on this image until I'm pulling my hair out. I need some clarification on a couple questions. Is the layers panel a complete list of every texture layer you used? Or were more stacked and merged together? I guess I'm bad at levels, because I just can not get the fine detail and even tone that you show in your last pic with some of the lines masked in. Also, how are the curve layers applied? Normal, linear light, etc? I assume you mean the dark line texture was the 30 px median separation, and the main line texture and paper texture layers are both 10 px median filter separations? My hierarchy is set up identically to yours, but I can not get the lines and texture to pop enough without a lot of high contrast defects also popping out. Please advise. 

[Benjamin-The-Historian]

Also, what method of masking would you suggest to mask in the dark lines?

Are there any parts of the top two texture layers that are masked off, or are they just stacked at full visibility on both?

[Benjamin-The-Historian]

EDIT add on: meant to say "I guess I'm bad at CURVES" not "levels"

(couldn't find a button to edit my reply)