They dont need to be OBD2 certified unless its an OBD2 vehicle. If its your 91, you should be able to use any cat , but the certified ones have more material inside and are more efficient- depends if youre having an exhaust shop do the welding, some of them comply with state procedures ( documenting the carb number on the cat ) .. others just weld it on.
Here in TX we just weld it on. When Cali emissions cars come in with clogged cats, we have to use an OBD2 converter, otherwise the catalyst monitors will be tripped.
RIP '94 Amber Fire Pearl Metallic Thunderbird on Bullitts [email protected]
96 Sport '02 Explorer PI with ported heads, 90MM LMAF, DirtyDog Marauder TC, Lasota tune, Jmod, Magnaflow mid-mount with X, no cats. Urethane drivers motor mount. HIDs!
I think you can get away with what-ever emmissions devices were originally on your car. Otherwise a majority of cars would need to be modified to fit CA stringent standards. But, I would check to be safe.