Searched the shed for a CB125 alternator, the first rotor I found looked like this.
What might have happended to that? It also looks like someone have been riding the bike for some time after that happened. Maybe it's the cause of this or the result?
Finally found a good one but the configuration of the stator did not match my Worshop Manual. Later found the matching wiring diagram in a CB125B6 manual. Anyway it's a 6V alternator and a flash light probably would be better.
If the magnets are good, I think I'll try to modify it and see if I could get more power.