So, internet sleuthing has led me to the rumor that S2000 torsens are the exact same as NA/NB miata torsens. Do they actually just bolt in in place of, let's say a stock automatic NA open differential? Axles fit? Spline count the same?
You are an enterprising group with many answers. Is it true? I ask, because we seem to be in some confluence of reality where S2000 diffs are cheaper than miata diffs. Who knew?
Found a post here saying they can be swapped in but you need Mazdaspeed axles:
https://forum.miata.net/vb/showthread.php?t=161418
Someone implying they've run an S2k diff in a Miata?
https://www.miataturbo.net/suspension-brakes-drivetrain-49/miata-torsen-vs-s2k-torsen-46105/
Someone else trying to put an S2k diff in a Miata, couldn't find MSM axles, looked into S2k axles but they're different:
https://www.reddit.com/r/Miata/comments/vw88xz/s2000_diff_on_miata_na/