It's a GameMaker:Studio source file, so you can change ALL the sprites. (Note that you need GMS1.4 or GMS2 to be able to use the source files)
If you have the Android port you should be able to run it on Android devices (I haven't tested this, but the engine mostly uses very basic functionality - the thing I'm the most worried about is the darkness effect in the underground levels, mobile devices usually have very low VRAM so surface effects like that don't always work properly).
There's no built-in functionality for weapon slots, a quick way you could implement this would be to have more "powerup states" that fires different projectiles (the example ones has hammers and fireballs) and add two new buttons that changes your powerup between all the weapons you have unlocked, and then change the health system to use a healthbar instead of the Mario powerup system.
