Import rules by country
Eligibility is a national question. Road registration is usually a state or provincial one. They are decided by different authorities.
Choose your country
United States
25 years from month of manufacture
Vehicles at least 25 years old are exempt from Federal Motor Vehicle Safety Standards, which is what makes right-hand-drive kei trucks importable without modification.
Canada
15 years from month of manufacture
Canada's threshold is 15 years rather than 25, which opens up considerably newer kei trucks than are legal in the United States.
Australia
Concessional routes under the Road Vehicle Standards Act
Australia does not operate a simple single age rule. Imports are handled through the Road Vehicle Standards legislation and its concessional pathways, and the requirements are more involved than in North America.
New Zealand
Entry certification rather than an age rule
New Zealand imports large numbers of used Japanese vehicles and the system is built around entry certification and standards compliance rather than a fixed age cut-off.
This is a summary written for orientation, not legal or customs advice. Rules change, and several jurisdictions have revised their treatment of kei vehicles in recent years. Confirm your own position with the relevant authority and your customs broker before committing money.