The first question is what are you measuring? In particular, what gearbox and motor will be driving those wheels? If you want easy to install and use, use encoders made for your gearbox or motor. The KoP chassis uses the TB-mini, which has a special encoder nub behind the drive shaft; AM sells encoders specifically designed to attach to it. If your gearbox does not have something like this, you may want to go directly from the motor.
Attaches to TB-mini:
http://www.andymark.com/product-p/am-3132.htm
Fits on the front of a CIM (shortens shaft, beware!):
http://www.andymark.com/encoder-p/am-3314.htm