For example, if I'm looking at a vehicle that's at least 25 years old but the seller put in a 20 year old engine, that vehicle is no longer eligible to be imported because US Customs now considers it a 20 year old vehicle. How have you handled verifying this regarding the vehicles you have imported from outside the US? Was it ever at all an issue?
This is an interesting twist. I believe we traded comments on FB about Schumacher. I have read of the government impounding cars for this and similar reasons (and that means crushing them). Mostly the discussion was on Defenders that had replacement engines. There might have been less than 20 or 25-year old re-VIN'ed Defenders that were busted because the newer engines gave them away. Enough of trying to scare you away but you may have to jump through one hoop on the CJ-8, not sure how many hoops on the Defender. The 2 important forms are DOT HS-7 and EPA Form 3520. I think you have an out for the DOT form but the EPA may need some work.
DOT Form HS-7
The first eligibility box allows vehicles 25 years or older. You may fail that with a newer engine if they know it's not original. The second box 2A permits vehicles that conform to all applicable safety standards and bumper and theft protection standards:
"The vehicle or equipment item conforms to all applicable Federal Motor Vehicle Safety Standards (or the vehicle does not conform solely because readily attachable equipment items that will be attached to it before it is offered for sale to the first purchaser for purposes other than resale are not attached), and Bumper and Theft Prevention Standards, and bears a certification label or tag to that effect permanently affixed by the original manufacturer to the vehicle or affixed by the manufacturer to the equipment item or to its delivery container in accordance with applicable National Highway Traffic Safety Administration (NHTSA) regulations [591.5(b)];"
Since the Scrambler was made in the US and this is simply an export version, they were made to conform to the US standards. My Overlander still has the original sticker stating that it conforms to all 1983 standards.
EPA Form 3520-1
This is probably the harder of the two, but you may still qualify:
"code E - vehicle at least 21 years old (calendar year of manufacture subtracted from year of importation) and in original unmodified configuration is either exempted or excluded from EPA emission requirements, depending on age. Vehicles at least 21 years old with replacement engines are not eligible for this exemption unless they contain equivalent or newer EPA certified engines. Customs may require proof of vehicle age"
You should dig into the replacement engine qualification and understand what kind of engine was put into the vehicle and whether it is EPA certified.
So, what kind of engine was put into it?