Your scan looks to only be missing a portion of the yellow/purple bars, so if the canvas was extended to fill 23" x 9", we should have an accurate scan.
Is it only the bottom portion that you didn't completely scan? It looks like you're missing a little over half an inch.
I thought the same until my fiance pointed out a discrepancy between the scan and the snapshot in this thread.
Going from left->right.
A: The Pac-Man section appears to be correctly positioned on the bottom and has the upper top cut off.
B: The two halves are mis-aligned (my fiance tipped me off on this, I would never have thought to look here. Once I saw this, the rest of it started to make sense).
C: The Galaga logo appears to have the correct alignment across the top, but has the lower portion cut off.
D: There isn't enough black space beyond the Galaga logo.
E: Gimp says the resolution is set to 72pixels/in. If we assume it was actually scanned at 300 (no matter how hard I try, my scanner does 299). Then we get a measurement of 22.623" x 8.45". That jives with emdkays estimate of 1/2".
It's all fixable. Took me more time to figure out where some of Gimps tools are than it did to realign the components and color them in.
Sorry about the cruddy quality of the attached image. I just wanted to show an example of where the misalignment occured, not remove the moire pattern from a shrunk image.