There are three possible solutions:

1) LIPO battery 7.4V
The lipo battery is capable of more current and lighter weight. You may need to add some lead weight to balance your plane. Although the battery is lower voltage (7.4v vs 8.6v), the lipo has lower internal resistance and is capable of more current. You will need a LIPO charger if you dont have one. check out www.unitedhobbies.com for some cheap batteries and Turnigy chargers.

2) Brushless motor
If you wish to keep you same electronics, something like the EZ-ESC with a brushless motor would give you the most performance gain. You would be adding a brushless ESC, a brushless motor, and the EZ-ESC. The cost is higher but the performance would be very high.

3) Larger brushed motor
A motor like a 480 size motor will add some performance, but not as much as you would think. Back in the day of flying the Aerobird challenger, changing from the 370 size motor to the 480 size motor did help - I would say about a 20% improvement in power. This in combination with the LIPO battery would help.

JC