Well if you go this route, mfactory sells shorter gear ratios for 3,4,&5th... There were 6 speed 1.8 civics made, the trans pop up on eBay sometimes. They were made in other countires but their 6th gear is our 5th gear so theres no fuel savings, just prob shorter gear ratios.
You're better trading in the car for a manual, too much work, time, and money. If you decide to trade in get the SI since it then has 6th, you get more power and vtekkkk, and lsd, and like webby said the 5speeds were only available on the lx trim so you don't have real rims, subwoofer, or sunroof.
The 5 speed is okay, if I shift too fast from 1st to 2nd it'll grind, rpms don't drop fast enough and it's not a close ratio gear box so if you try shifting fast the car will jerk when you let off the clutch because the rpms were too high. This was my first manual car and I love it but there are these quirks. Oh and it was a big learning curve to drive it smooth but someone said if you can drive the civic smooth you can drive any car smooth.