Solomiata.com is an excellent resource for simple, performance-enhancing Miata modifications:
http://members.aol.com/solomiata/index.html

"Hakuna" is a 1991 Miata with extensive, well-thought-out modifications. Luckily, Hakuna's owner documented the entire process.
http://www.pacificsites.net/~hakuna/miata.html

Robert McElwee has completed one of the most in-depth Miata weight reduction I have seen to date; his website fully documents the arduous journey of the "Red Beast"
http://ww2.lightweightmiata.com:8080/

SCCA SPEC Miata class racing has its rules and regulations...Here they are:
http://www.specmiata.com/specifications.htm

TECHNICAL ARTICLES

Informational Resources on the World's Most Popular Sportscar

The links below have been provided with respect to both the initial degree of engineering, and the intrinsic potential of the Miata as a tuning platform to better inform the enthusiast.