Not entirley sure how the Flush is done, I have someone else do the work, someone techy could answer, but maybe compressed air, or fuel oil mix blasted through.. Id get the barrel replated and a new head, And yes if you have the time strip it down and rebuild it, that way you can be sure its done!
of course you also need to find out what casued it and fix that too, otherwise itll happen again i would think.