Alright, done some digging. It seems like indeed it is possible to import an engine.
3.2 Engines Not in a Vehicle or Chassis
EPA regulates the entire vehicle, not individual parts, for cars, light trucks, medium duty
passenger vehicles (MDPVs), chassis-certified HDVs, and motorcycles. If an engine is not
installed, and is to be used in a car, motorcycle, or light truck, it may be imported as an
automotive part. Anyone may import a non-chassis-mounted light-duty engine for use in a
motor vehicle which is currently covered by an EPA certificate or will be covered by an EPA
certificate prior to introduction into commerce. No approval or Customs bond is required by
EPA. However, importers should be aware that several engine uses are violations of the Clean
● Mounting an engine to a chassis to "manufacture" a vehicle without proper certification
of conformity from EPA;
● Mounting a non-identical engine to a certified vehicle; and
● Importing an engine for use in an engine-certified heavy-duty vehicle or heavy-duty truck
without a proper label indicating that it is covered under a certificate of conformity, or
was built prior to 1970, or is otherwise excluded or exempted.
To import an engine for use in a car, light truck, MDPV, chassis-certified HDV, or
motorcycle, the importer must file with Customs, upon entry, an EPA Form 3520-1 and declare
code "W" on the form.