OK...easiest place to reference the relevant EPA regulations regarding motor swaps involving foreign-market motors not covered by EPA certification is in the memo linked below, which comes directly from the EPA itself. See in particular paragraph 7 under "A. Federal Law", which says:
"Another situation recently brought to EPA's attention involves the offering for sale of used foreign-built engines. These engines are often not covered by a certified configuration for any vehicle sold in this country. In such a case, there is no way to install such an engine legally. EPA has recently brought enforcement actions against certain parties who have violated the tampering prohibition by performing illegal engine switches."
Here's the link if you want to read the whole thing:
Regarding the claim that:
"A NON-EPA certified engine simply means that it has not been certified by the EPA and may be subject to emissions testing (where applicable). Now, if it fails the emissions test, then it WILL need to be brought into compliance (sometimes at considerable expense)."
Well, yeah, emissions testing is
part of getting a motor certified by the EPA. But it's nowhere near as simple as that. The process is spelled out here, again, in a link directly from the EPA itself:
Certification and Fuel Economy for Light-Duty Passenger Cars and Trucks | US EPA - https://www.epa.gov/ve-certification/certification-and-fuel-economy-light-duty-passenger-cars-and-trucks
Go ahead, dig into some of the requirements. It's a hoot. Not the province of mortal men. You're not going to do it.
Now, all that said, the OP has options if he wants to swap in a diesel.
Option 1: Swap in the diesel, register it outside the Colorado Emissions Program Area so that he doesn't need to have the vehicle inspected by CO authorities and take his chances with the EPA. Not a huge risk. I used to live in Estes Park. Saw a lot of elk, but never saw any Federal Agents snooping around looking for diesel swaps.
Option 2: Just live with the motor he has. Nothing wrong with it. No worries.
Option 3: Swap in the diesel
before he has the vehicle inspected by CO authorities and tell them it's the original motor. It
might work.
Oh, and this:
is kinda funny. There's
no Federal Exemption for "off road use only". It's a myth.
All of this talk of non-US motor swaps being illegal is regarding FEDERAL law, BTW. STATES are all over the place on this. Some don't care at all, some (like the Front Range of CO) are getting kinda picky about it. The reality is, the likeliest place for the Federal Government to notice that you've tampered somehow with the "original unmodified condition" of the imported vehicle is US Customs, when the car is being imported. After that, even though it's still illegal in the eyes of the Feds, no one is likely to be looking for it in most cases.
OK....Having derailed the OP's thread pretty much completely by now, I'm gonna bow out of any more "what's legal, what's not" talk and let the poor guy shower love on his Troopy.