You can always get muriatic acid from the hardware store and dilute it with water to reduce it's acidity.
Use outdoors only and be careful finding the right dilution - full strength will eat soft metals like brass and copper very quickly. - Works even better when heated.
In the end you will find that nothing can restore an old clogged raditor like a complete recore.
Once they are clogged, they can never be completely brought back to like new condition.
