For the exact physics, you could do a clean-room implementation.

Someone who is familiar with the disassembled JJ2 code should write a precise specification. Maybe someone working on JJ2+ could help you. Then, someone who has never looked at the code (!) should implement the physics from that specification. In this way, you do not infringe on the copyright of the original version.

https://en.wikipedia.org/wiki/Clean_room_design