now you've other issues.......
You see the corrosion. You're going to flush. And you might end up ruining the motor.
Know this....there are very small cooling passages around the cylinders and to the head.
If you start flushing, keep flushing, and keep flushing, and keep flushing. But you can't use a real strong cleaning agent, as it will attack the orings.
This is what happened to my MD7A. Cleaned it up after 20 years of not being flushed, it loosend the corosion, then the crud went to the smallest cooling passages, and caused the engine to overheat around the cylinders, causing the rings to take a set, loss of compression resulted. And the temp gage did NOT show the localized heating problem around the cylinders.
Then you'll want to rebuild, and throw more money into a great, but old, motor.
Then a $3500 overhaul, then 2 seasons later the new seals may have gone bad, filling the crank with water, ruining the engine. Thats what happened to mine.
key is being sure the engine is really flushed, and all the passages are clear.
And I don't have a clue how you can do that.