There are currently three Civic VX engines (d15z1) on ebay. If you have been thinking of getting one and have about $400 to blow now is your chance.
Two of them are newly imported from japan and one is from Washington State.
Here are the links.
I have one of these engines from japan. I perfer them over the USDM ones mainly b/c they have around 60k miles on them (if not less). I swear my engine is brand new. They usually come with sensors (not always) including the five wire o2 sensor which usually costs $130 by itself.
The Civic VX engine uses vtec-e which helps it achieve around 50mpg in it's stock form. With a few other modifications to your car I have no doubt that you can achieve 55+mpg.