[SwissJetPilot]

You might want to contact JonhhyFarmer as I seem to recall he's replaced the antenna amp. Unfortunately, from the pictures you've included, it looks like the threaded feature that the antenna screws onto is a significant part of the amp and probably can't be repaired.

The Workshop Instructions in the link you included are pretty straight forward, but it looks like a PITA got get to it.

 

[SwissJetPilot]

To avoid the risk of mixing bad parts with good, IMHO you'd be better off just spending the money and replacing all the parts.
Image from - aerial Audi TT/TTS Coupe/Roadster (ATT) 2008 year Audi EUROPA 35015

 

[SwissJetPilot]

Well done! That's an excellent cheap solution to what could be a quite expensive repair. I've added a link to this post in the
FAQ - The Ultimate Audi TT Mk2 (8J) Roadster Compendium under Section 7.