@MarkRipley I get that the requirements are insane, such that I'm keen to understand what slow, but steady, method might work. Time scales could be on the order of 1,000 years or more in need be. I guess another way to look at it – is it possible to put an engine on a planet and putter around to some other location in space, assuming time and cost weren't the issue? My constraint was that the engine itself shouldn't kill off the planet whilst it's doing its thing.