I think there is a series in California that uses them. I love the idea of them. Simple, light, fast motors that will stall and end your race if you do something stupid. That said, I think they are too hardcore for the US. If a series were to bring them in, they’d have to do it differently than they do in Europe. To start, it would have to be one make (karting isn’t big enough in the US for a local shop to support 4 or 5 engine brands) and the motors would have to be “stock”. At the CIK level, the OK motors are open to development and actual engine building. That’s not going to fly on a large scale over here.
That said, it seems that the X30 has stabilized to TaG issue for a while now. SKUSA, WKA, USPKS, F Series have all moved to X30 and people seem quite happy with them.