Clean up one vector image - probably half hr of work

Hi - I'm trying to modify a vector image of a flag. Currently the flag is divided by a diagonal line. I need this instead to be a horizontal line, cutting through the flag. I've included a mock-up of what it should look like. I'm not very good (or patient) with anchor points but it shouldn't be too hard if you're comfortable with Illustrator. In the attachment is the original vector (on top) followed by a mock-up of what I have in mind. Thanks very much!