You'll have to slide the coolant tank over to the front of the motor as well. And in the pic, it sure looks to be a 10mm. Look at it this way.. it's either a 10 or 13, and that pic doesn't look 13 to me.
On the tap with a hammer, you gotta put down the pink hammer your wife bought for herself and get a 5+lb garage hammer. Then it'll be a tap. You are in Virginia, land of no rust. How bad can it be. lol