Exhaust manifolds cracking are a very common issue on the 2000's. There is one company that "made" headers for the 100's. Doug Thorley in California. They made a run of 20, I am not sure if you can still have them ordered or made. But google them and give them a call.
I would do the side that has cracked. It's out of warranty, so replacing something that is not broken seems wasteful to me.