Yes the bearings do have to be greased witha high temperature grease.
It is a bit more complex than just "fitting new bearings", they have to be adjusted for end float using shims that go behind the inner race plus the outer races have fibre insulaters under them that come in a variety of thicknesses.
I would suggest you send your magneto to someone for a complete overhaul, get it done properly and you wont have to worry about it for many years.