Somehow I missed post #11 before. Sorry about that. The engine you are using is fairly small, so you will need to keep things light. Why not just use exhaust pressure as is usually done for your fuel pressure? There are also small, engine driven pumps (Perry pumps), but maybe not for that small of an engine. I would think that a constant pressure pump independent of the engine would lead to difficult starting (you would have to start with pump off) and additional weight for the batteries to run it.
John
John