Question of food sources for engineered life form to live in space depend purely to level of technology used in engineering such beings. 

If you have advanced technology enough, then you have strange living metal beings that can use any ambient energy sources to sustain themselves. Such beings could have organs that operate much like fission and fusion reactors to both providing energy but as well give ability for this entity to create elements it needs with these nuclear reactions.

If level of technology is less advanced, then you have clearly more biological looking beings that rely for energy sources more commonly used by living beings on our planet.