Can we have some more info?  Where are you, different states have
different rules.  Why did it fail, be specific please.

If you're tested with an exhaust sniffer take the car out for a run
before the test.  Hit the highway and run at least 15 miles at 65mph or
better, then get it tested. That'll get the car warmed up fully and
clean out the combustion chambers and exhaust system.

The early Miata's don't use an EGR valve. So if it failed for NOx make sure
the cooling system is in top condition. High combustion temps lead to NOx
failure. It is also a good idea to run a top engine cleaner through once in
a while to keep the combustion chambers free of carbon. Built up carbon can
raise the effective compression ratio and also lead to higher temps.
Alan

Generic catalytic converters, at muffler shops, are cheap
enough.  Since there's no EGR valve, that's the only possible
culprit.  At least that was the thing I replaced this year to
pass the Texas emissions test with my 1990 Miata.  The
numbers changed dramatically, from the first test (fail) to
the second (pass).  I'm in a "highly critical" area, suburban
Dallas/Fort Worth, with stricter standards than rural Texas.

Jim Stuyck

I have a 1990 Miata too, @ 165,000 miles.  Never failed a California smog
test.
Here's what I do to make sure I pass:
 (1) tune up - new spark plugs
 (2) very recent oil change
 (3) new clean air filter
 (4) fuel injector cleaning (if needed)
 (5) warm car up for at least 20-30 minutes before the test
 (6) new catalytic converter if necessary (I've never had to resort to
this)
 (7) Get Larry Dennstedt's advice in San Diego (Dennstedt's Auto Repair at
(619)-284-4911)
       Dennstedt is  a.k.a. "Mr. Goodwrench" with Miatas, also real nice
guy.  If anyone can get your Miata to pass smog, he be da man.  He wouldn't
charge you for telephone advice.  Good luck.