I would get a second opinion to be safe. It could be clogged passages in the engine, but it could also be clogged passges in the radiator. You might want to get the radiator flow tested to be sure.
In any case, there are chemical flushes that you can buy at the auto parts stores, but if you use one of these, make sure you follow the directions and get all of the chemical flush out. Your other option is to install a flush and fill kit from an auto parts store and flush the engine with a garden hose. If the problem is the radiator, I would just replace it rather than try a flush.
Installing a flush and fill kit will allow you to routinely flush the system to prevent this from happening again.