I am pretty sure that there are no licensing requirements whatsoever to fly single seater, flexwing ultralights in the US (and there are plenty of people over here crazy enough to try it without training).
However I had fixed wing microlights in mind, which in the US are being promoted as sport planes: Jabirus, Ikarus C42 and the like. By all accounts these are comparable to regular single engine piston stuff in performance and handling, which is why the FAA private certificate is considered good enough for them. You would want differences training of course, but then so would you if transitioning from a Cessna to a Piper. I am hopeful common sense will prevail in the halls of the CAA....