[kzrdude]

The data is hidden in the image data itself, it seems(!) Does this mean that an accurate screenshot can be used to clone the cartridge? For fun, of course.

[nacs]

Yes all the data (source, sprites, sound) is encoded in the PNG. You can simply copy the PNG to clone it (screenshot won't work though):

https://www.lexaloffle.com/bbs/cposts/un/undune2-3.p8.png

Also note that there is a "Code" button under the game window which shows the full source code.

[shakna]

Unfortunately, no, I don't think that's possible. There's a few null bytes used here and there, like marking the Lua offset, and your screenshot can't capture those, no matter how accurate the rendering of the actual bytes is.