If not mistaken, original is 2" from the extractor to the muffler. But don't bother to alter it into bigger size as it will decrease your power, if you like to have a louder or more aggressive sound, change out the whole system. You can try PM "Semperfi" as he have the whole new complete set of HKS muffler original from Japan for GTT. This might help you to get better HP and more aggressive sound.
I got a SR2.1 210cc engine fro sale. Come complete in everything from Greedy pulley tto Tmei exhaust manifold. Thngs that are not included in the engine are only Turbo, Wiring, ECU.