Welcome to nico with your GS 240
First off youll be in G-class stock, or GS as its known. Now in GS you can change the struts, as long as they use the exact attachment points and style as OEM, and dont alter the suspension geometry..
You cant add a strut tower brace in GS or any stock class unless it was a delaer option for that year. S13's never came with one from the dealer here, so no.
The front antisway bar on the other hand can be upgraded. So a whiteline ST cusco front antisway bar can be used. The rear bar cannot be altered..
Brakes can only be the same type and size as stock. Meaning brembo blanks (same as oem) from say tirerack are cool, but brembo slotted, or drilled are not. Any pad can be used.
For more basic info please visit, Stock class rules Keep in mind this isnt the official listing, but rather, a rather complete representaion of it. I would suggest beccoming a member, getting a rulebook, and following up on the scca fasttrack bulletins for exact info per classes. Also keep in mind that the rules state what you can do.... If you dont see that you can do it, you cannot...
SCCA forums Solo2 forum
These are 2 scca / solo forums I visit often. The scca forums are to the point anal about the books and will let you know that a shift knob, as it isnt stated that you can remove / change it in stock class, is illegal !!! It may seem like alot of rules but its tons of fun.
edit: BAH I got beat to the punch by 240_keyy.... I knew I would as he is another resident cone killer, but mines more informative

Oh yeah and by the way gunclap thanks for a refreshing first post. Im so tired of the same generic first posts we see day in and day out.