have you ever made a scale drawing? all you need is a reference measurement use milimeters or something. take measurements on the picture, then scale it to a good size for yourself. that's how I managed to layout my supercade copy before jeff was kind enough to post his measurements for it.
you should be able to to get reasonable close enough that it won't be noticable.
if you think about it, those buttons are about 1 1/4" diameter...shouldn't be too hard to generate your own plans