Are you talking about the two pieces of flat steel that go from the top link assembly back to about the midway point on the top of the deck (in a V shape)?
If so, then yes you can replace them with chains. But there's a caveat. If you are doing something to break one of those bars, then the rear end of your mower must be raising up quite a bit. If you replace the bars with chains, the back end of the mower will be free to raise up with no restrictions, but can raise up high enough to make the U-joint at the input shaft of the gearbox operate at a severe angle. This can destroy or cause premature wear on the U-joint.