Rare had many teams working siloed and independently. The team that made Diddy Kong Racing made a custom microcode based heavily on the official Fast3D with several differences. This microcode was updated and used in that teams next N64 games Jet Force Gemini, and I believe it was also used in their next cancelled game Dinosaur Planet, which later was released on GC as Starfox Adventures. I'm reasonably certain it was used in Mickey's Speedway USA as well, but I'm not certain.