Most cooking sites I see site the original source. I've also seen a lot of modified or adpated from XXX. If it's online, they include a hyperlink. If it's a cookbook, sometimes they link to it on amazon. If you are changing the recipe, you should be able to post you're version. I don't know the percentage of the recipe that needs to be changed though. There might be a guideline out there somewhere.

There are a lot of interesting articles about this. One I see a lot is that no one can own a recipe, but they can own the language of said recipe. So with that in mind, write up the directions in your style.

--Note this is all conjecture so take it as such.