Allowable mods in stock class;
Dampers (shocks 'n struts)
Wheels of the same dimension
Small wear items (filter elements, brake pads, battery, etc.)
Yes you can add all those things to an SRT-4 to make it faster, but you could say the same thing about the Focus as well...
Don't waste another minute waiting on parts, or anything else. Just get out there and start racing! You will suck at first, and you will be slower than many other cars, but the only thing you will regret is not starting earlier...