i'm pretty sure it needs to be an antimatter reactor

i'm sure it will need maintenance but part of its output will run its magnetic containment cell where the reaction takes place

once you nullify gravity, you don't have a speed limit problem, and collisions are impossible because they require mass and that is nullified.

of course i have no idea how you build such a thing but unlike fission or fusion, antimatter really does yield the e=mc^2 amount of energy out of any matter.