People have poured various types of acid into the water jacket, followed by a strong solution of baking soda to neutralize it. Then run lots of water through the system before refilling with a 50/50 mix of antifreeze and water. Muriatic acid like the type used to clean masonry, diluted with water to about 10% concentration works to soften the deposits and the flush removes it. Be aware that rusty freeze plugs might end up leaking from pinholes the acid exposes, and some of them can involve a lot of work to replace. That's particularly true of the one at the back of the block that's partially covered by the bell housing!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!