Well, Tanaka and Mitsubishi, among others, are EPA and CARBII compliant, and have been for many years. Keep in mind though, CARBII standards get tighter over time. Proposed changes will make them even tighter.
Jenna mentioned that the Zenoah uses an efficient scavenging system to limit pollution; Mitsubishi uses a similar approach also, so that catalytic converters aren't needed. The Mitsubishi would meet the proposed (future) standards as well - I don't know about the others. And, if needed, the engines which have been designed to utilize a scavenging system can also have catalytic converters added in the future, when the limits get even tighter. If an engine is only able to meet standards by adding a cat converter right now, what happens when those standards get tighter?
As augi talked about, getting an engine certified IS a lengthy and costly process. (Independent testing labs test the engines under various load and environmental conditions, and report the results to the governments.) Also, the testing is NOT a one-time expense. The company is committing to an annual testing and recertification process. This is why those engines do cost more than the inexpensive clones. Not only must they be manufactured to tighter tolerances, there are additional annual testing and certification/recertification which must be borne as well. Those costs get passed on to the buyer.
The upside of all this, for us, is that an engine which is built to tighter tolerances, and which is tested to meet those tight pollution standards for a specified minimum lifetime, is a more reliable engine, overall, than one which is not manufactured to tight tolerances and results tested.
