Hello,
I am afraid that 20 MHz is the closest bus frequency value you can get using the internal oscillator.
In case you need to get 16 MHz bus frequency, I recommend using an external oscillator.
The KEA128 supports 32.768 kHz crystal or 4 MHz to 24 MHz crystal or ceramic resonator.
Regarding the CAN bit rate, please refer to the Clock system at chapter 32.4.5.2 from the Reference Manual at the following link:
http://www.freescale.com/files/microcontrollers/doc/ref_manual/KEA128RM.pdf
Hope this information will be useful for you.
Have a great day.
David