Looks like you picked up a pretty good base for a kamm/boat tail!
Get the maintenance up to date and then start modding! The idle may be because of the cracked manifold. I wouldn't mess with the O2 sensor until after you fix the manifold, otherwise you might be wasting your time (and money).
Definitely air up the tires (40ish) and get an MPGuino.
Do a search on here for engine and tranny swaps. I don't know anything about the Civic line, but a lot of people are swapping between the different models. You might be able to swap a better engine and better trans and make a Franken-civic for the best results.
Also, I think you should put a spoiler on the back.
Civic Spoiler