once a plastic top aluminium radiator conks out, it is still best to change the whole thing. It is also best to put the same OEM coolant the manufacturer placed in your car from the production line to prevent "chemical reaction" with other coolant brands. but it is a different story for copper-based radiators as it can be repaired by most radiator shops. I don't like the idea of putting a brass tank top on an aluminium radiator, depending on how good it was repaired, it can still undergo galvanic reaction leading to corrosion and eventually failure. And you will have your radiator repaired again. Most radiator problems I know of starts usually when the owner changes the OEM coolant with something else he thinks that is better than the OEM....