I can't emphasize patience enough. I've seen this exact sort of thing completely backfire mostly because someone wanted the changes to happen NOW. Quite a few environments need the benefits of evolution, not revolution. And by forcing change today, it can make for a very unhappy environment for all.
Buy-in is key. And your approach needs to take into account the environment you are in.
It sounds like you're in an environment that has a lot of "individuality" to it. So... I wouldn't suggest a set of coding standards. It will come across that you want to take this "fun" project and turn it into a highly structured work project (oh great, what's next... functional documents?). Instead, as someone else said, you'll have to deal with it to a certain extent.
Stay patient and work toward educating others in your direction. Start with the edges (points where your code interacts with others) and when interacting with their code try to take it as an opportunity to discuss the interface they've created and ask them if it would be okay with them if it was changed (by you or them). And fully explain why you want the change ("it will help deal with changing subsystem attributes better" or whatever). Don't nit-pick and try to change everything you see as being wrong. Once you interact with others on the edge, they should start to see how it would benefit them at the core of their code (and if you get enough momentum, go deeper and truly start to discuss modern techniques and the benefits of coding standards). If they still don't see it... maybe you'll need to deal with that within yourself (especially on a "fun" project).
Patience. Evolution, not revolution.
Good luck.
