How to correct yellowed primer colors?

I have this primer whose cover and pages got yellowed by time. I scanned the cover and a couple of pages I want to put in my portfolio. I could manage to make the pages almost entirely black and white while preserving some gray areas of the illustrations. For this I first used IrfanView to convert the images to black and white, then used PP to remove moiré and noise.

But the cover is a challenge I have no idea how to overcome. I want to make the cover background white, the blacks to be 100% black, the grays to have their shades preserved and, most important, to preserve the green to black gradient effect in the title.

Would you please help me with that? TIA!