Japan has annual safety and emissions checks that apply to cars over two
years old (?), that are extremely severe, and result in many cars being
pulled off the road at a very young age as the cost of passing the test
gets too onerous.

The point of the regulations is to force domestic consumers to keep
purchasing new cars, ensuring that a domestic industry will exist. It
amounts to welfare, making those Japanese poorer who /don't/ work in the
auto industry, so that those who /do/ will get richer. It's a
redistributive tax, basically, and it prevents the government from having
to look bad by directly subsidizing the industry through general tax
revenues.

JDM items may have low mileage, but typically have spent many, many hours
sitting idle in heavy traffic. That "40K engine" might have *twice* the
running hours as an engine originally installed here.

Also, Japanese engines until recently had (still have?) significant
differences from USDM vehicles, such as carburetors, and mechanical fuel
pumps. This means that if you buy a JDM engine, you may have trouble
getting all your USDM cooling system, engine controls, and smog equipment
hooked up, and may have to blank off such apertures as one for the
mechanical fuel pump.

JDM transmissions may have different gear ratios to suit local driving
conditions, and may also have other differing features that may impede a
straight bolt-on attempt

Proceed with caution.