I'm making the source code to A Starspangled Zephyr available for free to anyone who wishes to learn how to code. ASZ was developed in Game Maker 7 back in 2010, but most of the code is analogous to Game Maker Studio 2. The bosses in particular are produced by some very neat code. 

As the 0.9 implies, this isn't the final source code (which is lost). So there are a few features missing, like the password system and level select. But the rest of the game is there.

Feel free to make your own mods, just follow CC-BY-SA rules and don't sell them.