There are special workflows to clean low quality BW or greyscale images, used originally to clean fax messages.

Principle is to use in bitmap editor combination of setup of layers and Gaussian blur. One or two times. And as last step a sharping. It works better if you increase resolution up to 600-1200 dpi first.

I attached sample after and original and several screenshots. I'm sorry I don't have English Photoshop but it should be clear any way.