You can go at this a few different ways...
1. The stock motor is already pretty "hot". If you opt to go with a 7.4v Lipo pack instead (check the
Compatibility List thread for lipo options), that's one way to upgrade
2. Atomic released a new motor for the Micro-T called the "Atomic BB". Supposedly the motor is about equivalent to the stock, except is uses Ball Bearings.
3. Go Brushless. This is the most extreme option as you'd have to swap in all your own electronics (ESC, Receiver, Servo) but there's been quite a few people who've gone this route and enjoyed the project.
If you're not looking to get too extreme, too quickly...I've recommend option #1 and check out the new offering coming soon from Team Losi directly (a Lipo and charger package).