The search for "Moto X3M" on GitHub reveals that the original commercial game (by Madpuffers/Flash) is not open-source in its official form. Instead, GitHub hosts:
The top repositories by stars/forks focus on playable clones that replicate the core bike-stunt gameplay. github moto x3m top
A community-driven toolset to track top records, create custom levels, and analyze gameplay for the browser game Moto X3M. The search for "Moto X3M" on GitHub reveals

So, why pair "Moto X3M" with "GitHub"? GitHub is the world’s largest repository of source code, usually reserved for developers building apps. Why would a Flash/HTML5 game live there? The top repositories by stars/forks focus on playable