The algorithm is a Yay0 if it is using the BIOS fonts. Try setting a Write breakpoint on the data and restart the game and start running again when the breakpoint is hit until you find at least 1 byte of the expected data at the memory address to find the routine that decompresses the font. You might be able to update chunk pointers in the file and create a bypass decompression hack to insert the font in the game. Your custom image must be in the original texture format and resolution for it to work properly from experience.
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