I used one of their standard gearboxes. I would suggest a mod .5 or mod .6 idler gearbox for a 5mm axle. I used their mod .6 23:1 gearbox but a mod .5 with a 25:1 ratio should work too.
Here is there website.
Be aware because their gearbox is shorter you will have to either angle the motor or add a belt drive to connect the motor to the gearbox. I got a smaller motor and angled the mount.
Pete