Hi Felipe,
I used the Tamiya Planetary Gearbox Motor
Tamiya part number 72001
Jameco item number 358213
You can find it for sale online at quite a few places, Jameco is one. You can alter the number of gears in the gearbox to give you more speed (fewer gears) or more torque (more gears).
There's a parts list attached to this posting, that may also help you with your project.
Good luck!
Don