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The most efficient engine/trans for your Civic variant is the 1988-91 HF engine and trans. Basically, exactly what you have. The trans has the tallest gearing (possible exception: California-spec cars), and the engine has a number of features that help its fuel economy. (It is an 8-valve engine vs. the other 88-91 CRX engines which are 16-valve, it may have low-tension rings, etc.)
The trouble is that they don't make rebuild parts for the thing any more. So if you can find a bona-fide set of HF pistons, rings, and bearings you should hold onto them. (Make sure the bearings are the right size group for your rods, though; there were several different ones and you don't want to mix them!)
You might be able to use a "JDM D15 three-stage VTEC" engine hooked up to your HF transmission. It will require some fabrication and electronics work to get the engine management to work, enabling the three-stage VTEC, but you might actually get better economy out of it than the stock HF if you tune it right.
Failing that, just about any of the D-series engines will bolt up to your HF trans if you use the right flywheel. And they'll all be pretty good on economy if you don't rev them up, which is where the HF trans will help.
-soD
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