Flushing seawater cooling system

[Windward mark]

My little yanmar ysm8 overheated and stopped last weekend so started investigating.

Found the impeller dodgy but with the weather getting decided it was a good time to pull things apart and check the general condition.

The engine starts well in warm weather but hard to start in winter, so I was thinking I could probably manage a valve grind, looking at YouTube it doesn't seem to hard.

I couldn't get the anode out so pulled off the top cover and found the anode was severely corroded and internally there is a build up of some sort of crusty substance (not salty)

So the question is, can I flush the engine cooling system somehow, I presume the hidden parts are just as bad if not worse.

While I'm asking for help, there is no resistance on the hand crank. I was going to pull the front off and try and see what's wrong.
Is there anything I need to be wary of ?

I haven't worked out to post photos yet, any tips 😊

Cheers, Tom