Marcus,
Start by cleaning the rust and rubbish out of the slot in the head stem (there is probably a key or peg to prevent the bearing from turning on the stem). With the stem cleaned up, add some penetrating oil, then using a piece of steel tube over the stem, try driving the bearing further on, before trying to remove it. (Much like trying to remove a rusty nail from a piece of wood, driving it further in usually makes it easier to remove!).
Regards,
Eddie.