Are you using it for grain or filling it? If it will only have to support its own wait plus snow, etc., you could use cable. Get something with a good strength to it. I would imagine swapping over with the right stuff wouldn't be cheap. If the rods were in good shape I would just keep them. They have found their level and curve by now anyway.
If you are filling it I would sure use the rods. Those folks used really good lumber and materials ages ago. They knew what they were doing and didn't cut corners. That's why she's still standing.