I don't think it's going to be easy to add parts to what you have without buying a whole original assembly. Or you could try coming up with your own solution with belts and pullets. Either way the motor needs a controller feeding some drivers/mosfets to give it power, and the controller needs to interface with your PC. There is the PWM2M2 project (the updated L2M2) -
http://forum.arcadecontrols.com/index.php?topic=160169.0 for this, along with a number of DIY solutions like the MMOS FFB project and some Arduino-based ones. I would imagine the FFB setup in STTC is identical to that in Daytona or Sega Rally, so you might try searching on here or google or even marketplace to see if anything crops up, whether that's ideas or parts.
Anything like this will have device descriptors for USB, and that will basically mean it will work with Boomslangnz's FFB plugin in driving games. I would strongly recommend proving that with a minimal setup on the bench, if you do decide to DIY, before spending a ton of cash or carving up the cab further
Pics of the Cab? I love the STCC cab, wish i had room for a pair of originals. And that there were any nearby. And that i had the money...