@Kenn0n - Just Ral is fine.
First off, am I right in assuming you don't own a drawing tablet?
In any case, you should also remove the text from the lineart, just zoom in quite a bit and focus on the text alone - try not brushing over any lines.
Then, you make a 2nd empty layer (on top of the one with the pattern redraw), and you use that layer to draw the missing parts yourself.
Now... there's a few ways you can go about "filling the blanks":
1. Just pick some shade of gray for your Paintbrush, and draw slowly.
[" and "
]" can be used to adjust your brush size easily.
Try a brush tip with 80-90% hardness. In places you think you drew lines too wide to match the original art, just use Eraser (with similar brush tip hardness) to delicately cut them down to what you think is right.
You might also try using "apply jitter", since the original lines aren't smooth, but you'll have yo do a bit of testing with that to see if you can achieve a similar effect.
2. Basically the same as above, but instead of Paintbrush, you can use Clone again.
You just need to pick a source with a big area of original ink - in this case, hair below the neko hairpin.
Set alignment back to "none" and let go of the mouse button frequently to start from the source again (the hair source is still quite small).
Alternatively, you can select the hair from the background, duplicate it to a new layer, duplicate that layer a few times, them move them about to make a bigger source area, select all the hair layers and merge them into one. Put that layer above the one with the pattern, but below the one where you'll draw the lines. After you're done with your lines, you can just remove this layer.
3. You can use Path Tool to draw the lines and fill the gaps with stroke.
It makes it much easier to draw curving lines, but it'll be harder to change line thickness. I won't help you there, since I'm rather unfamiliar with GIMP.
Or, you could just choose slightly thicker lines and just use Eraser wherever you need...
Afterwards, if you think the lines are too smooth, you can try applying some filters to the line layer. For example filters>noise>spread>1&1px (or 0&1px or 1&0px)... Or whatever you think looks best.