I hate to break it but you can't mod something for "payment" its illegal because you would be redistributing something as your own. The concept is confusing but pretty much you can't edit game iso's for money purposes.
That is depending on what the company wants you to do with the game.
The only time its ever legal to make a modification for a game is if its purely for enjoyment, not for money purposes to "buy", or "sell" modifications. Bootleg's are another story but those are illegal as well. That is if its being sold as an original copy just edited to make money.
Also depending on how popular the game is, you may see fan made games across the intrenet, or mods that change how the game works like ssbm. Which is where your trying to head towards (modding.) There are ways to replace the stages, or add alternative ones depending on your knowledge of replacing textures, music, character models, etc. But a lot of time has to be taken into it, and I don't really find it worth a lot of my time since it can be time consuming, and frustrating. But if you really want to you could try looking into Brawl Box, and try looking into the ISO using other alternative applications like gcrebuilder.