For anyone else wanting to know the answer to this question, the answer is...
While the heatsink will fit quite easily, installing a 120mm fan is not easy because it will hit the memory dimms when placed at that side of Ninja. With some tweaking you can place the fan above the dimms, but you will then deprive the bottom of the heatsink from airflow. It needs this airflow because the bottom of the Ninja is a conventional heatsink. All-in-all I do not recommend it.