This is the kind of research that takes time and lots of effort to pursue, so maybe over time we can figure it out.
I think it's pretty clear that the Epicurean material in Cicero's "On Ends" and "On the Nature of the Gods" was probably never lost to the world for any considerable length of time, and I get the impression that Diogenes Laertius, which has the bulk of the core stuff, was generally available too.
We seem to get the impression in talking about the recovery of Lucretius that the atomic theory was brand new upon its "rediscovery," but if Diogenes Laertius was available (as appears so) then the core of the teaching was available through the Letter to Herodotus.