Remove the circlip with circlip pliers. This then allows the bearing to be knocked through the casing from the inside.
I'm lucky enough to have the original Piaggio tool to drive the bearing from the casing. You can also use some sort of steel pipe as a drift to knock it out. If it is very stubborn you can heat the casing to release it's grip on the bearing.

Once the bearing is out, a new 6204 bearing can be driven in to the casing. Heating the casing is a great help before driving the bearing in with a socket or similar thing.

Make sure the bearing seats all the way past the groove for the retaining clip. Insert the retaining clip and .....

...fit the oil seal firmly in place. Sometimes a large socket is helpful to gently tap it all the way home.