Because these drawings are black and white and contain no little to no grays, they respond pretty well to the brightness and contrast tool by increasing the contrast 100% a few times. I would do this once or twice and if the black lines started losing chrispness, I would duplicate the layer and set the top layer's blend mode to overlay and then flatten the image and increase contrast again. I also ran the noise/dust & scratches tool at radius 1 & threshold 2 to get rid of some left over dirt.
This reduces the majority of the smudges quickly. Because there will tend to be some tint to the white areas, I tend to use the healing or clone tools to hit the left over spots because erasing or painting with white will tend to make spots that can appear when your print that may not be visible on the screen. It just cuts down the work. If you are scanning tracing paper, it will help to place flat bright white paper behind the sheet prior to scanning as the white scanner lids seem to reflect shadows onto the paper.
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After Pic1
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Before Pic2
Linear Mode
