So, I decided to add the ATASCII character set to the code. This is a hand-tweaked version of the original, which I obtained from the ROM using an emulator and a BASIC program to print out the values. The display is 8 rows x 7 columns and most of the ATASCII characters are 6 x 6. The ATASCII data are stored row-by-row because the ANTIC graphics chip pushes the bits to the GTIA display chip as television raster lines. I had to transpose the individual character bitmaps because the demo software from Sparkfun displays as columns for easy horizontal scrolling. I eliminated some blank columns on the narrow characters to make it a bit more proportional, which improves readability on the scroll. I also nudged the lower case characters up or down a line to look better. Finally, I replaced the Sparkfun logo bitmap with a Fuji. The code is on my GitHub.
Here's the final product:
The music is by Adam Gilmore from the Atari 8-bit game Zybex.