Almost every e-motor will work but I prefer this tiny ones out of a CD/DVD driver. They work at 1.5 Volts and the rotation speed is not to high. The magnet I use on the shaft of the Motor is also demounted from a CD/DVD-driver (you can find 2 of them close to the lens).
First superglue one magnet to the shaft of the motor. Do not glue shaft to the housing else it wonÃÂ´t work anymore.
Secondly mount the motor to the bottom side of the rack. I used hot glue. The magnet should be close to a test tube hole. If you have a different looking rack then you have to find a place near a tube and fix it there.
Put a magnet in the tube you are about to use. I used a, 1cm (~0.4inch) in diameter and 1mm (0.04 inch) thick, neodym magnet. But you can also use the one out of the CD/DVD driver.
Do not use bigger and stronger magnets or the glass will crack.