if its not legally registered/insured and/or you dont have the required license, then yea its really down to where you live and how people see slightly noisy machines, and finally if you encounter cops, if they dislike or happen to be into such things. here in my area, people ride quads down the street (very suburban, no trails to go) and dirtbikes and i ride my minibike and people just look, smile, maybe wave. the one cop i rode past didnt care at all, i had a helmet and other gear and wasnt going fast. i also stay off any major streets.
making something street legal is ALOT more work than just trying your luck, and ALOT more expensive, but it also allows you go go anywhere and no one can prevent u from driving around. proper tires ($), wheels ($$), brakes, engine ($$$), exhaust($$), registration ($ hundreds), insurance (more $ hundreds), riding/driving gear ($$$), and anything you may need to do like a license or safety course ($$$$) like CA requires for some riders, etc..
requirements vary by state, so $ totals will also vary but its not like you'll get by with a couple hundred and by legally on the road.
the EPA thing as an example is, as far as i know, for the entire US and mainly deals with chinese engines and utility/industrial engines. anything legal as an on road of offroad bike or car engine is EPA approved, so nothing to worry about there. on the other hand, most of those ebay 110cc or 200cc lifan or other chinese motors [while they may be exact copies of street legal motors] are NOT EPA approved for cost reasons and cannot be registered in many states (CA for sure). the same goes for utility motors like briggs or HF blue motors or honda utility motors and so on. legal to operate in a machine, not a vehicle.
often times, small details are overlooked and people register custom made things just fine, but it's not a rule by any means.
