i'm sure it's been said a million times so far but i cannot stress how important it is to kee the weight to a minimum. you could try drilling out holes in all large pieces of metal but just enought to not compromise structural integrity.
the engine could also do with good old fashioned tuning which woudl help it make the most power allowing it to be as fuel efficinet as possible.
i know it is a long shot but you could also look into turbo charging as it is a very efficient way of getting more power from an engine. all you'll need is positive intake manifold pressure so even a tiny turbo pushing 1psi would be sufficinent (no need to rework the internals).
Good luck and well done on your effort so far. do let me know if i can be of any help at any time.