There is a difference with California legal and Federal legal emission requirements. However if this 2005 Ford Focus ZX4 is equipped with all the Federal required emission components (in other words, the emission system has not been tampered with) it can pass the California smog check without needing any modification.
The smog check rule in California states any "used" vehicle which is equipped with the necessary Federal Emission Components is eligible for California registration as long as the vehicle's exhaust emissions fall within California's limits and the vehicle's emission system has not been tampered with.
So the two questions you should ask the seller of this Ford Focus is has he/she ever tampered with or modified the smog parts, and what overall condition is the engine in? Best bet is to ask them to provide you a emission inspection report from Missouri and compare those results to California maximum allowed emission limits for Co, HC, and NOx (the three harmfull pollutants the smog check looks for).
California Emission Cut Scores
