You need to be able to be winning or challenging for the win at the highest level events in North America if you plan on competing in Europe. You’ll also need lots of money because not only is KF expensive (that’s why it’s basically only factory teams participating), but living in Europe will cost a pretty penny too.
You’ll also want to get into a 2-stroke kart as soon as you can so you are familiar with the machinery. A 2-stroke TaG/KF is a completely different driving style and philosophy than a 4-stroke.
I’d say once you’ve proven you’re capable of winning at events like Florida Winter Tour, SKUSA Pro Tour, Rotax Grand Nationals, SKUSA SuperNationals, and USPKS in the TaG class, then you’re ready to take the step to try and race in other parts of the world. Until then, keep racing and trying to improve.