I suppose that you could heat-set it, depending on the type of ink you used...
It's just normal ol black pen, nothing special, cheap if anything.
Would a fine-point sharpie work? I've seen lots of people use sharpies here but I've never tried it. I suppose it wouldn't look as 'drawn' as it is, but it's far better than busting out my paints and attempting to trace all my fine lines with a brush (that'll never look right!). Is there a way to seal that or is that just permanent?
sharpie tends to bleed a bit on fabric, much moreso than a regular ballpoint pen. Also, with a lot of wear, sharpies will fade away. if you know what type of pen it was, you might be able to go to the store and see if that type has waterproof ink. (it varies by brand)