First of all: was here is short for [irgend]etwas, hence your translation to something is correct.
Einfallen can have different meanings, but happen is none of them. The right meaning here is come to mind.
I don't see how you got from your word-translations to the last sentence, a more literal translation than We'll come up with something could be
Something will come to our mind.
Schon, according to Duden can express emotional empowerment, but has no literal translation in this context, so let's just ignore it for now.
I am not sure if I answered your question properly, please let me know if you have further questions.