EMP energy can possibly gained by trying to put the 2 poles of the supermagnets onto eachother on high-rotating speed. The energy-burst that should follow must be 'caught' and used to give the spaceship speed. Because this happens constantly under high speed,the spaceship should get energy-bursts so quickly,that the spaceship will constantly have a very high average speed. Ofcourse, we must be able to control and manage the entire proces,so we can also slow down and stop the spaceship.
Also check Rotating Core Gravity