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I think a basic 10 micron (Racor or similar) water separator filter in addition to the engine fuel filter is adequate. I would not add an additional filter.

If the gas is drained, fuel/fill/vent hoses replaced and then you use treated gas, you should be okay. I recommend treating the gas every time you fuel up.

I think a carb cleaning will get you back to whole.

With blended fuels, I am not exactly clear, or maybe it's "not comfortable" with storing the tank 3/4-Full over the winter. I expect there will be a good discussion on the board on how should deal with winter fuel storage as the time gets closer.

Post the results from the yards look at the engine.