Fender pickup replacement?

There's nothing impossible there, as long as the replacement pickup will physically fit in the space (don't do something crackhead stupid like try to stuff a humbucker in there). You *do* want to make note that if you put in something *too* much higher output than the neck and bridge pickups, and you rewire that 5-way, the middle pickup will tend to overpower the neck+middle and bridge+middle selections, making them less useful (you'll end up with 3 effective settings - neck, bridge, and 3 in between that all sound pretty much like middle).