Here are the EPA regs for the US. As long as you have a 49cc you are exempt. (even in CAL but they will tell you different) Also there is an EPA 1 time personal use exemption. You can build and run anything you want just once and you can't sell it for 5 years. Heck you can build a Harley with open pipes and use that 1 time exemption. It called the "personal use exemption". Most states do require a manufaturese certificate of origin but in Oregon we also have a DMV form for "assembled vehicles" that are not any manufactured model. I used a PUCH bycycle frame, this can be used to bypass the origin cert. but you must sign another form saying that you have all required equipment and meet all NTSI and EPA requirements. SOme kits now come with a tiny catylitic converter in the muffler, mine has one, you can see where it is in the muff cuz it runs so hot it melts the cheap paint off the muffler. Works good though no smoke at all.
These 49cc motorvehicles are Exempt from EPA Emissions regulations under the Code of Federal Regulations;
Title 40, Chapter 86.407-78 Par. (b) and
Title 40, 86.401-97 Par. (b).
Therefore no EPA Certificate of Conformity is required for the registration, titling or operation of this vehicle. (for emmissions but safety equipment is required)