My professional background includes circuit design.  A strong magnet next to a conductor will cause some effects even if everything is rigidly mounted (see the Hall effect), and will certainly induce voltages if things are vibrating relative to one another (that's how a motor works, kinda).  But the effects are going to be pretty darn slight.  It might be an issue if you were doing sensitive measurements with your electronics, but you're driving a motor, and that's one of the most electrically noisy things you can do with a circuit, short of driving an arc welder.  So you're almost certainly safe.

My professional background includes lots of different kinds of design.  Say "absolutely, that's a safe thing to do", and 99 times out of 100 you're going to be right.  But that one out of 100 wrong answer always bites deep.

So do a test: set up a timer, ESC and motor on a bench (I'd put the motor in a test stand with a prop to load it).  Get a humongous magnet, at least twice as big as the one you plan to use, turn the whole mess on, and wave the magnet around the ESC and timer -- see if anything misbehaves.  If it doesn't, then that 99 out of 100 goes down to maybe 999 out of 1000, or 9999 out of 10000.  At that point, you've reduced the possibility of failure due to the magnet/electronics interference below the probability that a line will break, or that a local duck hunter will mistake your plane for a bird and shoot it out of the sky.  At that point you can go flying with as much confidence as ever.