mass spectometer and relativity

Hi, I had a homework question in my physics class that I'm not totally sure about. We're supposed to design (not actually build, just explain on paper) a mass spectrometer that can measure the speed of electrons going at .998c, so taking into account special relativity. Here are my questions:

Would it basically be the same as a normal spectrometer, same design?

How would you account for relativity? I think you would assume that the mass would get bigger in your reference frame, so you would have to divide by that lorentz factor to get the object's rest mass. Is this right? What about length contraction? Do you just multiply r by the lorentz factor?

Also, how would you actually set up the magnetic field and the electric field?

Any help would be greatly appreciated. Thanks!