This is probably a legitimate option.
In one place, EPA literature says: "
EPA regulates the entire vehicle, not individual parts", but further along in the same paragraph it says:
"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." I would interpret that to mean that only an EPA certified motor can be imported as a complete motor if the intention is to install it in a vehicle. But parts, they don't worry about too much. So if you send over a head, and a block, you're sending parts. If you send over an assembled motor, it would technically need to be EPA certified. Stupid, but there it is.
It could be argued that disassembling a motor only to reassemble it once it arrives in the US is somehow intentionally circumventing the spirit of the law, but in your case, where the motor is literally the original motor that came in the car when it was new, I can't see how that argument would get very far. When you're done, you'll have a car, over 25 years old, with the original motor in "original, unmodified condition", which is exactly the requirement for the EPA exemption. What more could they want?
Another option requires a somewhat literal reading of EPA form 3520-1. Near the top of the form, in the introductory paragraph, it says:
"This form must be submitted to U.S. Customs and Border Protection for each motor vehicle (including motorcycles, disassembled vehicles, kit cars, light-duty vehicle engines) imported into the U.S."
Notice how they've included "light-duty vehicle engines" in the definition of "motor vehicles". If that's true, then any 21+ year old motor could be imported under the same exemption as a complete vehicle, by checking option "E" on the form.
All of this is to say that EPA is somewhat less than clear about the rules re: importing complete motors that are over 21 years old (and thus should be eligible for an exemption from EPA regulations). That specific scenario is never spelled out clearly in any EPA documents I could find. What I did find was an EPA information office, which claims to be able to answer such questions. I have contacted them and await their response. I'll update here if/when they get back to me with definitive info. Might be too late for
@mudgudgeon but it could prove useful for future searchers.