Water heater replacement - advice?

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Speaking of valves on the engine jacket water loop...

I added a normally-closed solenoid valve on the upper hose. The valve is only open when the engine is running (the "ignition" key is on.)

Before this, my starboard engine block stayed warm and the water heater cooled down over time, through natural circulation. It's been very effective.
 
Looking at anode I decided to remove the drain valve to check to be sure of the fitting size. Well I was not able to unscrew it, even with much effort the valve seems to have been ultra tighten with a kind of paste. As I did not want to break it apart I did not try much "exotic" way of removal. So now my question. If am not able to remove this drain valve is there another way to setup an anode to the water heater?

L
 
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