So I have been researching the crap out of this lately. I'm sure you have found information about the 25 year rule. Older than that, pretty easy. Newer than that, not so easy. Not impossible, just more work.
The basics of it, if you can contact the manufacture and get them to check and see if the car met US crash and emissions, it SHOULD be as easy as a filling out some forms and taking that letter. If the car doesn't meet those standards, it's a whole bigger mess and probably not worth it.
Mind you, I have not done this yet, nor talked to anyone that has done this either. And I haven't gotten to the level of trying to call anyone at the US customs. I'm not sure if they will give advice, but it may be worth trying.
But this site gives some good information, websites, and those forms.
https://www.snowbirdadvisor.ca/how-p...-car-us-canada
If you do figure out something, I would love to know what you find!!