Since Filoni never got to finish the Fett vs Bane episode of TCW it’s not impossible that we’ll get to see it in a flashback. There’s already been some speculation that the “Book” in the title refers to different chapters in Fett’s life and that it will be mostly or partially about his past. Daniel Logan, who played young Boba in AOTC, could play a young Fett and with a bit of re-writing or maybe some subtle digital de-aging they could pull it off.
With the technology being used, i would say they would de-age Temuera like they did in Aquaman, i good thing about it is, they wouldn’t even need to do it much, he would probably have the helmet on for about 80% of the show.