Engine buildup for MPG?

[Garrett76zt]

Hey everyone, I've been wondering for a long time now. . . if I was to select a motor and then build it up the way I wanted to, what kind of things would I do for good gas mileage.

Engine selection:

Lets say I'm going to choose one of many different types of engines that a certain manufacturer made, lets say Honda. What are the important factors? Least displacement possible? DOHC or SOHC? In regards to bore x stroke, is undersquare or oversquare desirable? What else is important in the initial selection of a motor? Keep in mind this question is independent of engine management, just the physical properties of the engine itself.

Engine Build:

Now that I've decided on a motor what factors help with MPG? Within reason, aka 87 octane, is higher compression better or worse? Would there be any reason to increase displacement? What about ring gap? I would think that anything you could do to decrease rotating mass would be a good thing, aka pistons, rods, crank, flywheel, etc.? In regards to windage, the less friction the better, so perhaps a polished crank? Crank scraper? etc?

Ok I know alot of questions here, but if you were to build up a motor from the ground up how would you do it?