I would be concerned with vinegar (an acid) doing further damage to the metal in the block. A base like Drano (Sodium Hydroxide) should clean stuff out of the block without doing further damage to the metal. It is strong stuff though. Take care to avoid getting it anywhere you don't want it.
As mentioned earlier, CLR is designed for the purpose of getting rid of mineral buildup. That's probably the best thing to use.
Of course I don't know what the crud is. Maybe it's biological? I'd say Drano might be more effective in that case.
Whatever is used, be sure to rinse very thoroughly.