Don't feel the need to maintain the structure or phrasing of the original. If you are working with an existing article on the English Wikipedia, you will likely find the two articles have a completely independent structure. Even with new articles, don't feel you need to make your translations conform to the original article.
If portions of the article appear to lack good sources, use your best judgment as you translate those sections. Go ahead and leave out unsourced material altogether (or provide references of your own).
It may be necessary to add material explaining terms or cultural concepts.