The center seal is what they call a 'Labrynth seal' which uses a series of oil passages to seal the motor. It uses the oil injection pump to do this so keep that oil tank full. As such, there isn't much to wear out.
If you feel the need to replace it, the crankshaft needs to be pulled apart, replace the seal, and have the crankshaft pressed back into place. Intruders and air cooled motors use a regular center seal and since they are inside the crankcase, they are pretty protected from the elements.
When someone decides to remove the oil injection for racing, or just because they don't want it, On the invaders it is necessary to change the crankshaft to an Intruder crank, or leave the oil pump in lace and feed only the center seal.