In this section I'll go over how to fix up an image when you can't actually clone. When there isn't any clone source, there are two tricks to "redrawing" the image.
The first trick is copy a portion of the scan and paste is as a new layer, which you can then rotate until it fits what you need. This is particularily useful for curves (where you can copy a portion of the curve near the missing area and rotate it slightly so it fits), and lines (where you copy a straight line and rotate it until it's at the angle you need). Let's begin:
Copy the area, paste it, and rotate it into position. On the picture to the left (below), I've added a red stroke to distinguish the new layer. On the image to the right, I've taken off the stroke so you can see how the pattern no longer matches up.
Once you have the new layer in the position it needs to be in, use the eraser tool to erase everything on the layer but that line you need. Finally, select the layer with the scan, and clone in the surrounding area.
What's nice about this technique is that you don't have to worry about the new line when you're cloning (since it's on another layer). Also, a word of caution: Rotating images will blur them. Never rotate a new layer more than once, and try to do this before the image is resized to minimize any noticable burring.
The next trick is just an easy way to redraw lines without having to worry about ruining the image. In the image below, it's necessary to redraw the shadow line after deleting the sfx text:
To do this, white out the text, and then create a new (empty) layer. On the new layer, draw a line in the general shape of what it should be:
Now, since the new line is on its own layer, you can edit it freely without harming the rest of the image. Zoom in and use a hard eraser to chisel away at the line until it's the correct shape:
If the line is the right shape, but too dark, use the Dodge Tool to lighten it. Clone in any missing area on the scan layer, and you're set!
Head to the next section for more tips on redrawing.