Hi Frederick,
It's actually a bit of a job because you have to be able to extract the outer race of the drive end bearing from the magneto body. Getting the armature etc out isn't too bad - just be sure to remove the safety spark gap screws before pulling the thing out.
If you're going to replace the oil seal, you'll also need a new insulator to go behind the bearing race, and you may find you have to do some shimming (usually behind the inner race of the contact breaker end bearing or between the magneto body and the contact breaker end housing) to obtain the correct endfloat after the changes. So all in all, not terribly hard, but delicate and it needs tools, as well as a few parts.
It's worth saying, though, that magnetos will quite often operate with a fair bit of oil in them! So it may be there are some other factors that could be contributing to the problems you have got.
Good luck,
Bill