I'll guess it is her vent, from the feet and the poop. You probably wouldn't notice the umbilicus if it healed normally -- and they usually do.
If her poop is as wet as it looks in that pic, you don't need to clean it. It's dried poop that blocks the vent that is dangerous.
Sorry you lost the others.
Be sure it has ample clean water -- that may take some close management at first. And be sure it isn't too hot. Give her something to substitute for a buddy. A small stuffed animal, a feather duster, even a rolled up old sock.
If there is any way you can get a couple more chicks (of any breed) or can find her another home with chicks, she would be better off with companions.