What you really need to do is scan it, then convert the scan to a vector image, and create g-code from the vectors. Try Inkscape, it's free. You'll want to get a better .dxf exporter than the one included with Inkscape. Try this one. Big Blue Saw - Big Blue Saw's DXF Export For Inkscape
You should be able to use LazyCAM to convert the resulting .dxf to g-code.
Personally, I much prefer to redraw the part in a CAD program. You can load the scan in and trace it. This will give you the most control, and the most precise parts.