As that other thread says, you need a 7 inch diff head (that's measured by the crown wheel size).
Afaik, all the 1.8 MX5 diffs are 7 inch. The trick is working out which ones have the Torsen diff though, not the open diff. I prefer Torsen type 2 over type 1, they just look more robust. I prefer clutch type LSD over Torsen, as you can rebuild them and "tweak" them.
Note, some MX5 are 3.9, 4.1 and 4.3, but I've never been able to find a 4.3, and no-one seems to know how to work out which cars had them (if any in the UK market)
Here's a useful link for IDing Torsen centres: http://www.miata.net/garage/diffguide/