Raw water cooling - removing scale
Vinegar, dilute acids will all work .... however they will also remove metal. Better to use products: Mar-Solv or Ryd Lyme to dissolve the scale but not the metal componentry.
Take a good look at the engine''s thermostat..... a raw water cooled engine should have a thermostat that opens at 135 degress and not 180 degrees. If you find a 180 thermostat, the internal passages of the engine are probably also severly ''salted'' and you will definitely need to ''pickle'' the engine. Check the Thermostat .... or the engine temperature gage.