There is a difference with 49-state emissions legal systems and 50-state (California) emissions however a USED 49-state emissions equipped vehicle, such as in the case of a 2017 Fiat 124 from out-of-state, can pass the California smog test and receive DMV registration. Note, a new vehicle on the other hand, can not be registered (or pass smog) in California if it is not California emissions certified.
The smog inspection for 2000 and newer vehicles consist of a visual and functional inspection of the vehicle's engine emissions system. So long as the engine's emission system has not been tampered with, the check engine light is not on, and the emission monitors are ready, the vehicle can pass the smog test without any emissions modification; no smog parts need to be added. Since the Fiat 124 you plan on buying is model year 2000 and newer it will not need to undergo the tailpipe portion of the California smog inspection.