Unless you have a blaster, I would clean that up with an angle grinder w/flap wheel and then try to assess how deep the pitting/scaling goes. Those tubes are somewhere between 1/4" and 1/2" thick (sorry I don't know), so it's not like a little surface rust affects the strength. From the pics, the worst problems are out towards the wheels, but those backing plates can be easily replaced with a disc conversion.