Honestly, I would cheat.
1) You already have it in Photoshop, right? Use the Magic Wand tool to select the white area. Under Paths, click on Work Path and save it as Path 1. Export that back to Illustrator.
Open your original and lock it. Open the new exported path and copy that to the locked original (to use as a trace). Then tweak the curves on your new path until they match the original.
2) Or alternatively, it's a very simple shape... lock the original and redraw it with the Pen tool. Should take you all of five minutes.