Would it be fine to modify them?
According to the included file "
GrafxKid's publicly released pixel art assets fall under the CC0 1.0 Universal Public Domain License.
CC0 1.0 terms - https://creativecommons.org/publicdomain/zero/1.0/
Which means you can do ANYTHING you want with them. Dude is super nice about it, I really appreciate it :)