Are you planning to make a ROM hack? If so yeah, those would be allowed since they aren't just changing graphics and music.
That being said, if your hack and up being that simple I don't think that many people would want to check it out or would rate it very high. If you want to know what kinds of games you'll be going against you can check the entries for the previous game jams we hosted from below:
https://itch.io/jam/mfgg-fangame-jam/entries