It is the engine. If the engine was out of a 2010 car, its a 2010 engine.

Nukem
Have a Zetec in my mk2 with twin jenvey TB, built in 1980 passed ADR27a getting sign off next week by engineer. Did take 8 hours of dyno time and 2 epa tests but finally got it passed. 

That pretty much hits the nail on the head.
 
You need to meet the ADR standards of the engine or vehicle body, whichever is later. At no point does the ADR state HOW and under what configuration of engine components need to be used. The crutch comes in proving ADR compliance, and that requires time and money in order to meet the standard(s) and prove you meet them. I note throughout the entire thread that no-one has mentioned just how much each of the EPA tests cost in order to prove you meet the required ADRs. Anything can be achieved if your pockets are deep enough.
 
Best option. Dedicated Liquid LPG Sequential Injection. But that's another discussion...
 

Andrew, that isn't right when it comes to emissions.  The 'which ever is later applies'

Depending on the other mods you may require an engineers report regardless so it may be an idea to discuss with an engineer. I've learnt that no matter what you here or have been told it all comes down to the interpretation of the person you enlist in regards to the legal requirements. Speak to a few engineers and get it from the horses mouth.