Shocks can be replaced/changed with certain limitations.
Sway bars can be added or removed.
Reading the rule book or at least prinitng the section on what's permitted (beginning with the book's pg 60)would be beneficial.
Attending an autocross and talking to other competitors is helpful especially regarding tires and tire pressure.
The most significant improvement in your times will come from experience (seat time).
On the link below scroll down to the second heading
Solo/autocross for an educational
forum
http://sccaforums.com/forums/default.aspx
Also below is the North Carolina Region/SCCA
http://www.ncrscca.com/