Hello Richard,
I do appreciate your patience.
Please review the information below:
The latest/last updated sensor fusion v7.2.x is part of ISSDK released through MCUXpresso ecosystem.

Sensor fusion sources for supported platforms e.g. FRDM-K64F-AGM01 are released under BSD-3 clause open source license that allow external users to use, modify, distribute as long as they keep NXP copyright in source files. If you need more information, please get in touch.
In the SDK package, under middleware/issdk/algorithms folder you may find the source code.
I hope this information helps.
Regards,
David