AFAIK the Ultra is a modified G510, so any bike that is made for the G510 will work with the Ultra. Correct me if I am wrong. It requires new tooling for the frame, as soon as the demand for the frame gets big enough more bikes will be on the market. I have seen bare frames offered to home builders but not many.
The main "limitation" being that it's kinda a little bit illegal in many countries and provinces / states. Look at the current class system in the United States and you will see it has very little support for anything above 28 mph. It has a peak power of around 1500-1700W and can get you up to 32-35mph, which is small moped territory. So it's also illegal to ride it on the street in almost every country. I question ebikes with this much power. Worried hot dog riders will have an accident on a bike path killing a walker, kid or elderly cyclist. Government will impose new regulations.