Would you please be kind enough to explain to me what hapened to the Torah that Itzchak learned with Noah’s son and that Abraham teached? It is not clear to me if it was preserved. I believe it was Oral (not written) so I ask myself if we just forgot it or it is a part of what Moses received in Sinai?

The Rokach in his explanation of the siddur, writes that Shem and Ever were expert only in the seven Noachide laws. The Ramban (Bereishit 37,3) writes that the Torah that was studied in the Beit Medrash of Shem and Ever were those abstract ideals that are at the basis of the Torah. The Torah as we know it, includes and transcends therefore what was studied then. R. Yehuda HaLevi (kuzari 4 25) among others, attributes Sefer Hayetzira to Avraham Avinu.