I just sold my 1985 crx hf (to get a HX). If I'd kept it I'd have shielded the recessed headlights which were a source of drag (or swap in the whole front assembly and valance from an '86'87). Blocked the grill. Extend the front air dam (the stock from lip is only about 1/2" deeper than the dx or si). It already had a passenger side mirror delete from the factory.
As for motor swap, thats ALOT of work to get another motor in there that gets the same fuel mileage. I wouldn't be surprised if you had to modify the frame due to the crank pulley hitting it.
btw, I was getting 47-49 and I was NOT hypermiling at all.
It was interesting that the owners manual said rolling down your windows would hurt your fuel mileage. That's fine but a black interior in a car with no a/c and a shape with lots of windows that makes it a moving greenhouse in the summer is just not conducive to keeping the windows rolled up. The dash would get so hot in the summer that only hot air came through the vents
