Dude - stay away from EPA and all other agencies not involved in the process...! They have bugger-all to do with the import! (For that matter, you are importing an engine "for off-road use only"...!) There are no "EPA agents" sitting in the customs office!
Any way, when you get the call from the shipping company that the goods have arrived, you simple march your happy-arse over to their office (NOT a governmental office of any kind) and you ask for the paper-work "to go and self-clear the shipment!" (Get the customs office address...)
You take the paper-work to the designated CUSTOMS office and simply state than you would like to clear the shipment and hand them the papers. They will ask you what is in it and you will tell them - tweo used spare engines just like what is in your car now. Period - no EPA involvement!
If one of them gets nosy, you will simply state that the motors are for use in an off-road application only, get your papers stamped, pay the duties and leave!
Take the stamped paper-work to the warehouse, collect your crate and ride off in to the sunset, never to be heard of a again!
Perform the most factory-looking installation that you can muster and get the car smogged. If at all possible, get the smog done on the old motor first!