'Light' is made of a variety of frequencies that depict color, from Infared(unseen) to X-ray(also useen)and all the colors in between. now, if it is possible to 'slow down' light and have that energy captured as a fuel for use in an engine, it should be possible to move the mass at the speed the frequency that you are using for the fuel. Theoretically, couldn't you move beyond the speed of light by combining two of these energy releasing frequencies at the same time?
I don't think I fully understand you. Anyway, there's not all that much "fuel for use in an engine" to worry about. For an idea of what light might be useful for, see: Solar sails.