(I'm talking about stuff I don't understand, so feel free to ignore me.) Space isn't entirely empty. There are a few hydrogen atoms hanging out here and there. Imagine if a spacecraft was flying really fast, and it was collecting those tiny few. It could either use a massive funnel at the front of it, or it could use something electromagnetic. Once it collects them, it could use fusion to release energy. Then, on the other side of the spacecraft, it could shoot out the output as hard as possible.
Ad maiorem Dei gloriam inque hominum salutem.