Upvote:2
Here are some basic resources for you. Most of these were originally written by Indian scholars and now exist only in Tibetan or Chinese translation, although the BDK America collection also includes texts written by Chinese scholars.
Tibetan texts (translated to English):
Chinese texts (translated to English)
I assume you are only looking for secondary texts, not Buddha's original discourses, but if you are into alternative versions of these, then Sutta Central has a nice collection of non-Pali versions of suttas, mostly Chinese translations from lost Sanskrit or Gandhari originals (with English translations).
Upvote:2
I don't think you'll be able to find "lost" texts translated into English. An example of a recently-found text that used to be "lost" is the Gandhari Dhammapada.
This article, https://www.ancient-buddhist-texts.net/Buddhist-Texts/C3-Comparative-Dhammapada/Comparative-Dhammapada.pdf, compares several different Dharmapadas. The Gandhari one is one of them. There is also a translation of it published by Timothy Lenz. This is only one example of such a text. There are far too many to list individually.