I'm honored and humbled that you want to publish an adventure for this little game. Of course you're free to do it - the game itself in under CC-BY license, so you just need to mention that it was made for Endure by Fred Bednarski somewhere in the pdf. A link to this itch page would be nice too, but not necessary.

...and not sure if you seen my response to your previous comment (it was a bit late :/) but feel free to make any changes to the rules to make your adventure play as you want it.

I originally planned to write some adventure modules on my own (but life got in the way and I haven't published anything in years) and have them pretty much self contained - adding all the mechanics needed into the adventure module itself. If that's something you'd like, so people don't need two pdfs - go for it.