phosphoric acid at a 5 parts
to 1 part acid will leave your tubes sparkling. Mix enough to dunk the whole tube bundle and leave it until it stops bubbling, remove and rinse with copious amounts of fresh water. If there are any other small deposits left redo the process. When you have finished and reassembled the heat exchanger
(with new "o" rings the dirt in the acid will have settled and you can pour it off into a safe plastic container for reuse or dispose of it in an environmentally safe way. And yes check and replace any zincs as required.