interestingly enough Its possible to all that, look at Brawl+ for example, it rebalanced the entire game, and with that you also got additional character costumes, so its very possible to get eg Curse of Darkness Dracula over the Judgement one, but there has to be people willing to model and rig for judgement
the the judgement models are ripped and usable in brawl, as are the stages, which suggests a similar format, just the games have different animations and battle systems to work around, trust me I had thought of fixing judgment
to mod the game all you need to do is have riivolution installed and know how to make the XML files for that and the raw source stuff, Models music, stages, etc, I'd suggest wii explorer to explore the file system of the dvd disk