Having built the Printable Marble Machine #1 which is hand operated, I wanted to add a motor ..... which led to the redesign of quite a number of parts.
The motor which I settled on is a Pololu 298:1 Micro Metal Gearmotor which attaches onto my plastic mount using the Pololu Micro Metal Gearmotor Bracket. This motor runs off 5V and rotates at 45rpm - just about as fast as I think the design will cope with. It is powered using a 5V DC mains power supply which came with a mobile phone - I replaced the original power connector on the power supply as I wanted to use a 2.1mm DC power jack plug and socket set.

I have uploaded my rather poor video attempt here: http://www.youtube.com/watch?v=YOaXsGhuKLQ

Thanks also go to gzumwalt - he has made some cracking motorised models which motivated me to have a go at this one.

As well as the original M3 screw set - 25mm screw, two washers and a nyloc nut, you will need the following:
Pololu 298:1 Micro Metal Gearmotor
Pololu Micro Metal Gearmotor Bracket
2.1mm DC Jack socket
2.1mm DC Jack plug
5V DC power supply (I used one from an old LG mobile phone)
M3 nut and M3 grub screw for the sprocket gear.
Heatshrink tubing - always handy!

The bracket comes with screws and nuts which enable it to be fixed to the motor mount - attaching the motor and bracket to the motor mount should be done before you slot this sub-assembly into the base.

Print each of the parts which I have uploaded. Four of them are the same as the thing uploaded by JasonM621 : the spiral chute, chute brace, chute guide and the Gear_Lift. The other files are the ones which I have redesigned to accommodate the motor. (Or in the case of the entrance chute, to improve the way in which the ball bearings are captured and to prevent them from jumping out onto the floor).

Careful inspection of the photographs and videos here and those provided by JasonM621 should be all you need to figure out the assembly but things to watch out for include making sure you have the Gear_Lift part facing the right way - the side which lay against the print bed should be facing towards the inside of the machine otherwise the tilt of the holes will be going the wrong way.

Be careful when wiring the power supply and make sure that you match the +ve and -ve wires to the plug and socket. Some testing will need to be carried out to ensure the right polarity to rotate the sprocket gear in the right direction.