A five HP barstool racer is fine, but I have some students in my automotive class who are building one with a 440 Yamaha snowmobile engine in it! It should be a nice death ride, with all the scrapes cuts and chaos that a guy loves.
You don't need any forms. If the company sends it UPS then UPS makes you pay all the government fees. If the parts come from a private person, the customs will look it over and send you a "please pay now" in the mail. If it comes by mail from a company, then the post office collects the...