Can anyone help me with modding this game https://gyroscopik.itch.io/micro-gamer-shinmy

I wanted to modify the character sprites to be someone else, but I don't have much experience with unity. I asked the creator and even offered to pay. They said they were down, but it's tough to get a hold of them. I would appreciate any help with this. Also, I saw them do a version where they replaced the custom game with Doom, but they kept it personal since it had a few bugs. If you know how to swap it out, please let me know. Thank you so much.