Both parts of the sentence are in passive form. So in that sense the subject is implicit, if that's what you mean.
You've pretty much got the English translation down. I'd write it as something like
"Once you cross the threshold, it will definitely be easy to find sponsors".
Notice that I'm using the "you"-passive voice here as well. "Kun se(n) kynnys ylittyy" means rather that "Once its threshold is crossed". So the difference is essentially that "kun sen kynnyksen ylittää" implies that once you cross the threshold there is something that you are now capable of doing, in particular that there is interaction from someone involved.
On the other hand, "kun se kynnys ylittyy" is regular (progressive) passive which means that there is not necessarily human interaction involved, but the consequence could occur by itself, for example.
"Kyllä" emphasises the fact that it'll be easy to find the sponsors. You could also translate it (in this context) as "it would surely be easy to find sponsors" or something like that.