All you need to know about MX5 diffs:
90-93 manual 1.6 Eunos Roadsters were fitted with viscous limited slip diffs; Mazda never called this a LSD, but a sports diff.
Most, but not all, 93+ 1.8 Eunos Roadsters were fitted with Torsen differentials (4.300 final drive T2 from 1996).
Diff upgrades are very common.
The usual places to source a used diff is any of the MX5 specialist breakers; MX5heaven, Autolinkuk, PJ Sportscars (Shardlow) etc. Most Mk2 UK 1.8 MX5s will have a Torsen diff fitted. From 2002, a weak Fuji SuperLSD was used on Sport models.