Do you, since
you are solely and only fixing "badly writen English"
have to be yourself familiar with the underlying source from which
it supposedly comes.
My opinion is that you need to be sure you understand it before you
change a paraphrase.
I don't want to seem to argue against paraphrasing or
copyediting, since they're both clearly vital. But yes, if
you're going to copyedit, you do have to be pretty scrupulously
careful that you don't change the meaning! Just now I came
across a fine (counter) example. Read <http://en.wikipedia.org/
w/index.php?title=Ian_Michael_Smith&oldid=213296701>. Ask
yourself: in which movie, Simon Birch or The Mighty, is the hero
afflicted with Morquio Syndrome? About which movie's audition
were Ian Smith's parents initially wary? Now read the preceding
(prior to someone's copyedit) revision <http://en.wikipedia.org/
w/index.php?title=Ian_Michael_Smith&oldid=210232536>, and answer
the same questions.