Forks flex. To some degree this is normal. Before making any rash decision about the one you're worried about, I suggest you look at some other bikes and see if their forks flex as much.

When braking, does the bike pull to one side or the other? I ask because of the following train of thought:
--if the fork is compromised by internal rust, it is presumably worse in one leg than the other. I doubt both legs are badly rusted to exactly the same degree.
--if only one fork blade is so badly rusted that its strength is compromised, it will flex more than the other.
--if the right fork blade flexes more than the left, the bike will pull to the right when you hit the front brake. Does it?
If not, then I wouldn't worry about your fork.