a bottle of radiator flush stuff should help a bit, follow its instructions, then rinse that out and fill with a strong (50-70%) antifreeze mix, repeat after a month if it looks rusty again, that should be all it takes to get looking decent again. look into your radiator, if you see chunks of scale, take it out and use your garden hose to back-flush it and dump out all those chunks.
i know thats a common thing, as my 87 diesel has chunks of scale sitting in the radiator where i can see them, waiting for me to clean it out.
even if you do replace the radiator, run a bottle or two of rad flush through it to clean out the block and head