are you sure you have water in the tank? what are the symptoms?
in my opinion there are really only three possibilities for water in the tank:
- it was there since you got your freewind and you never noticed because you never run your bike on reserve
- it was in the fuel you got at the gas pump
- your tank cover is really the problem
so before you start replacing things make sure you find the causing problem...

my tank cover for example is also rusty becuase my bike was a 'standuhr', which means it was standing arround for 12 years in a garage without petrol...
so back to your question: you can try to remove the o-ring and then get a fitting, petrol-resistant one and try to put it back on, if that doesnt work you probably have to replace th whole thing...