All german warbirds I know of seem to have their current formal reg painted on somewhere too even if they are otherwise painted in the original colours.
Just make sure: The display of swastikas is formally (and seriously enforced) forbidden in Germany (except for very few "historically required" museum or movie a/c with a special permit) .
But that shouldn't be a prob on your Auster.
If you have a permit from your "local" authority for your paintstyle it shouldn't be a problem. Maybe there's some fineprint what to do on international flights?
This is the Luftfahrtbundesamt (german CAA) phonenumber and their public affairs spokesperson's. Maybe you can ask them.
Telefon: +49 (0) 531 2355 0
Fax: +49 (0) 531 2355 750 / 751
Pressesprecherin des LBA
Öffentlichkeitsarbeit Cornelia Cramer 552
[email protected]