if you don't overheat, no need to do anything expensive like changing the rdiator. the rust may be coming from the block, the radiator, or the waterpump. coolant has anti corrosive properties so it's a good idea to always use it after flushing the rad/block a couple times.

aluminum block engines don't normally rust, but cast iron block engines do. you can also try overhauling the rad, mura lang naman ito. then flush again until the water clears. then put coolant. and when you flush, make sure you disconnect the intake/return hoses from the block to the rad so the water from the block comes out (it will look rusty). don't forget to put them back before you start.