Chicken Run - Source code (September 7, 2000) (Gameboy Color) : Free Download, Borrow, and Streaming : Internet Archive
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Source code (presumed to be the final version), design documentation, and 52 builds for Chicken Run on the Gameboy Color, developed by Blitz Games and...

The documents are a bit broader, encompassing other platforms’ Chicken Run games (including an unreleased GBA title).