US Customs is going to be your main concern. Then worrying about if the company is legit or not. Hope you don't plan on giving them your credit card# or bank account info. What happens if the engine is a lemon? They ain't going to ship you a new one and pay to ship the old one back.
There are a couple of 4hp diesels for sale on Ebay. Probably a hundred more, but a safer bet.
As I stated in my earlier post, you should start with a frame designed to handle the torque and weight of the engine.