Hi Johndo!
Thanks for playing the game and happy to hear you like it :)
You are correct - enemy sprites will not be loaded by the game if you bought it on Catalog - when I was getting it ready for publishing on Catalog I did a few tweaks, one of them was to make the game load faster on start. I compiled the sprites into the internal 1-bit format that the game actually uses, before it was done when the game was starting up (this saves about 10-15 seconds on startup). I did it because I haven't really seen anyone trying to modify it and I figured most people just wasnt that interested in modding it.
But Im super happy to hear you wanna have a go at it! If you send an email to me (redterrorplaydate@gmail.com) with your name and Id be happy to give you a build from itch that supports loading the sprites so you can modify them.
I should probably mention this somewhere on the playdate catalog site for others as well to avoid confusion. I wasn't quite sure how to approach it so I just didnt mention the modding at all on the playdate catalog page.
Sorry for the confusion!