Here is the rule in colorado:
(407.0) ENGINE CHANGE:
(a) FOR THOSE VEHICLES IN WHICH THE ORIGINAL ENGINE HAS BEEN REPLACED, THE EMISSION LIMITS AND APPLICABLE EMISSIONS CONTROL EQUIPMENT FOR THE YEAR AND MODEL OF THE VEHICLE BODY AND CHASSIS, AS PER REGISTRATION AND TITLE OR REPLACEMENT ENGINE, WHICHEVER IS NEWEST SHALL APPLY. FOR THOSE DIESEL-POWERED VEHICLES WHICH HAVE BEEN CONVERTED TO OPERATE ON FUELS OTHER THAN DIESEL' THE EMISSIONS LIMITS AND APPLICABLE EMISSIO CONTROL EQUIPMENT FOR THE YEAR, MAKE AND MODEL OF THE GASOLINE-POWERED ENGINE EQUIVALENT AS ORIGINALLY MANUFACTURED, FOR THE VEHICLE BODY AND CHASSIS, PER THE REGISTRATION OR REPLACEMENT ENGINE, WHICHEVER IS NEWEST, SHALL APPLY AS DETERMINED BY THE EMISSIONS TECHNICAL CENTER PERSONNEL OR DESIGNEE AND SPECIFIED ON AN OFFICIAL EMISSIONS PROGRAM VEHICLE EVALUATION FORM DR 2365.
(b) FOR 1975 AND NEWER VEHICLES IN WHICH THE ORIGINAL ENGINE HAS BEEN REPLACED, IF EITHER THE VEHICLE BODY OR CHASSIS OR ORIGINAL ENGINE, AS PER. REGISTRATION AND TITLE OR REPLACEMENT ENGINE AS MANUFACTURED HAD A CATALYTIC CONVERTER SYSTEM, AIR INJECTION REACTION SYSTEM, MICROPROCESSOR BASED AIR AND FUEL CONTROL SYSPEM, AND. FUEL FILLER NECK RESTRICTORS, THESE EMISSIONS CONTROL SYSTEMS MUST BE PRESENT, INTACT AND OPERATIONAL-BEFORE A CERTIFICATE OF EMISSIONS CONTROL MAY BE ISSUED.
(c) FOR THOSE VEHICLES TITLED OR REGISTERED AS MODEL YEAR 1975 AND NEWER, THAT WERE ASSEMBLED. BY OTHER THAN A LICENSED MANUFACTURER SUCH AS KIT-CARS, REGISTERED AND TITLED ACCORDING TO 42-6-107.5 AND 42-6-117 C.R.S., AND ASSIGNED A COLORADO IDENTIFICATION NUMBER, THE APPLICABLE EMISSIONS CONTROL EQUIPMENT AND STANDARDS WILL BE BASED UPON A DETERMINATION BY TECHNICAL CENTER PERSONNEL OF THE VINTAGE OF THE VEHICLE ENGINE. AN AFFIDAVIT MAY BE ISSUED BY THE TECHNICAL CENTER PERSONNEL AND THE YEAR OF THE ENGINE SHALL BE PRESUMED TO BE THAT STATED BY THE VEHICLE OWNER UNLESS IT IS DETERMINED BY TECHNICAL CENTER PERSONNEL OR DESIGNEE, AFTER PHYSICAL INSPECTION OF THE VEHICLE ENGINE, THAT THE YEAR OF THE ENGINE IS OTHER THAN STATED BY THE OWNER.