I should do a tutorial on this. It's been asked so many times before, but here goes.
First mark the magneto rotor (magnet) so you put it back on the same way. Remove the center nut and remove the rotor. This is where it gets sticky. There is a woodruff key that sometimes rocks and binds in the keyway making getting the rotor off hard. Ace Hardware sells a faucet handle puller that works perfect for removing the rotor. What you don't want to do is pry too hard on it with screwdrivers. The crankshaft is small in diamter at that point, about 5/16", and can be bent by unequal lateral force.
After the rotor is off, pick out the old seal with a seal pick or a small diameter tool. Clean the seal bore, lubricate the new seal and the shaft and drive the seal in to the bottom of the bore with a seal driver or a socket of the appropriate size.
Reinstall the rotor and coil and you're good. The coil might or might not be salvagable. It might be so oil soaked that it is shorted. New coils are available at most vendors who sell bike engines and parts. Good luck.
Tom