To ensure that cars on Japanese roads are safe and up-to-standard, a strict and comprehensive vehicle heath check test is performed under a law known as Shaken Law. This is performed every 3 years to ensure the vehicle has not become dangerous for the Japanese Domestic market and that the smoke, exhaust and overall vehicle condition is still good enough to be driven. Most drivers however prefer selling their cars prior to having the inspection as they know the underlying charges for the inspection and what it could imply if the car fails the test.
So if you are in the market looking to purchase a JDM engine, make sure to do your research and get as much information about the seller as possible to guarantee that the engines they are offering are ACTUALLY imported as JDM engines from Japan. You could view website of the seller and look for customer reviews and feedbacks to ensure you are making the correct call when choosing a specific seller. Due to the over popularity of JDM engines, small car repair shops, resellers and vendors also know about the high demand of these type of engines and therefore would do everything possible to snatch a customer who is looking for them even if they have to falsify or fake certain facts. The best idea is to ask for import documents which are usually sent along with the engine once it is shipped from overseas. These import documents or other sale invoices would give you an idea about the authenticity of the JDM engine that you are purchasing.
If you are still in doubt about the deal and whether or not it is an actually imported JDM engine, you could do further investigation to validate if the seller is registered among the US border protection and US customs website. If no such details are available, you would be better avoiding purchase from such a seller even if is having good reputation.