This is from my 1985 Civic service manual (which doesn't cover the Si). The only other 5th gear I know of is the 0.655 from the HF, which is what I put in my Wagon. You could change the final drive, and get more pull in every gear. Or just drive in 4th gear.
1985 Civic Sedan:
1st = 2.916
2nd = 1.764
3rd = 1.181
4th = 0.846
5th = 0.714
R = 2.916
Final = 4.266
From other sources around the internet (I can't guarantee these are correct):
1985 CRX Si = 4.428 final drive, other gears are the same.
1986 CRX Si = 4.400 final drive, other gears are the same.
an interesting tidbit: the 4:93 FD from the 1980 Civic 1300 5-speed is a
compatible final drive for the 85-87 Si (and other) transmissions. this
was the low-buck way to get a short trans in the old-school days.
unfortunately, 1980 civic 1300 5-speeds aren't too common in the junkyards,
making this a tough piece to find.