Great work, Tape! I love the colouring and, especially, the lineart here. :]

The main trick is this. Scale the thing up using cubic interpolation to make it double-size. Then use the threshold filter to decimate it until you get most of the basic line structure and all the other crud is gone. Using the Curve tool my help to clean things up before you do the decimation. Then you go over it, rubbing out all the little bits that shouldn't be there and joining up the breaks, redrawing over the lines where necessary to clean them up. As you can probably tell, I had to do a fair bit of guesswork around the collar which is why the vest was wrong in the first version. I assumed it was a medallion not a zip.

Anyway, I've done something like this before, but mostly with inks. Never tried it with a sketch before.
With inked line-art, the width of the line increases of course, but you can still end up with a higher effective resolution. In the case of Salomere, my restored version was higher resolution than the original!

looks awesome~ =3