As far as I know, the Fratelli Tanfoglio TA90 is very similar to the Tanfoglio Witness, just a different stamp/importer and the slide mounted safety, made by Tanfoglio of Italy. Correct me if I am wrong guys. Here is a exploded view of what I believe is your pistol Link
and I think the part you need is a Sear Housing Pin.
Here is the Fratelli Tanfoglio website
directed to the pin you need but it says it is not available for the U.S. market. You may have to have someone manufacture the pin for you...