Here are the most likely causes for your 1991 to fail emissions testing. Not necessarily in correct order
1) Leaking fuel pressure regulator (easy check, moderate replacement)
2) Bad oxygen sensors (should set a code, front easy, rear not so easy)
3) Catalytic converter (sounds like a marble in a coffee can upon cold startup, but goes away when engine warms up...muffler shop repair if you don't have the tools)
4) Dirty/Clogged EGR (very easy fix)
5) Leaking fuel injectors (moderate check, easy fix once you check)
6) Dirty internals, engine oil -and/or- wrong grade fuel. (easy fix)
I don't know what your local laws are but in my area, you are allowed only ten days from failure to retest (assuming an expired inspection sticker).