When a person converts, he chooses a hebrew name for himself. That as i understand will be his forename and his surname will be Ben-Avraham (in the case of male). This is the name he will be called up for Aliyah to HaTorah? Can it be permitted that n ger change his surname that appears on his ID/Government furnished documents through registering it through the propper government office as another hebrew name i.e Ben Yehuda instead or is this not allowed? I understand though that Ben Avraham will always be his name for all religious activities/events Thank you

Shalom! He is definitely allowed to choose any last name he wishes- like Ben Yehuda. or HaYisraeli or whatever he wants! It's even a good idea to choose a Hebrew last name, to feel and be seen just like any other Jew, and many rabbis actually Hebraicized their names when making aliya, to feel that much more Jewish-Israeli (like Rabbis Neria, Yisraeli, Goren, Yashar, Ra’anan, and many more), as I did myself (from Chwat to Shvat)! You are correct that the halachic name for aliyot to the Torah, ketuva for marriage etc., will be Ben-Avraham. Although technically, it might be a little confusing to have an halachic name: Ben Avraham, and a similar but different last name: Ben Yehuda, but it’s fine halachically, if he wishes. With Love of Israel, Rav Ari Shvat