Sure it’s possible. But it isn’t 'easy'. It would be relatively easy at this time to do the following operations:
Replace model textures
Replace item names
Replace font
Replace music
Changes would probably require a moderately experience hacker.
[Also, depending on the exact workings of Capcom’s banhammer, changing the quests could result in getting banned online. I highly doubt they are extremely rigorous in that regard, but there’s no way to know exactly what they test without checking. Also note, online quests are likely transmitted by the server, and so not modifiable. The evidence for this would be the online event quests (small chance of this being mistaken).]