The spaceship was just an ice chunk floating in space, until the protagonist captured and hollowed it out, put engines and equipment inside, and started it moving.
The heat from the engines melted the ice around them, which froze again in space, but things, including the engines, kept moving around inside the ship, and the computers worked overtime to continuously recalculate the center of mass and the thrust vectors to keep the ship on course.
The protagonist was an independent merchant and also a member or collaborator in a rebellion against central/Earth government, and the payment he always required was spare parts and computers, or methods to increase the computing power of his current ones.
I read it in the late 80s, and the friend who lent it to me had gotten it as a gift a few years before, so that's why I assume it's from that time. Maybe late 70s, but I doubt it. I seem to remember the cover was of a big chunk of ice with an exhaust trail passing near a planet, gas giant I think. The lettering was in the futuristic style of those years.