I have had big problems with this. If you find the right customs agent I am sure it is possible but if you are so unlucky you get the same customs agent I had, it might give you a headache.
An engine should be brought in with a HS-7 form, the same to import a car. On this form it gives the 25 year EPA exemption rule for cars. For just engines though, there is no excemption rule mentioned on this form. It clearly says all engines have to meet EPA.
After a month at customs they released it because they finally beleved me (or they didnot beleve me but were sick of me calling everyday
) it was an original replacement engine for my 1981 Land Rover. I had to prove though that this car was mine by giving them a copy of the title.
If you do it, try to find someone who can give you an invoice and have them print a producion date of at least 25 years ago, on it. This might make it a little bit easier.