Sorry, Sandra....... I didn’t understand.
Try this:
https://www.worthpoint.com/wor...o-porsche-1859417520
OK, so that one only gives you what it is worth. 
I would still google “Gus Mozart vintage license plate frame” and see what pops up. Places to look are eBay, thesamba.com and the porsche356registry.com
It appears that a lot of people hang onto them as where the car was originally bought, but I bet your story and name would do a lot to pry one loose from someone who cares.
Good Luck hunting.
Edit - Come to think of it, try contacting Gary Emory at PartsObsolete.com to see if he has one or two in his parts stash. He’s West Coast, but you could also try Adam Wright at adam@unobtanium-inc.com as some of those made their way East, too. I don’t know Gary, but you can use my name with Adam and make sure they both make the name connection between your Dad and you. LOTS if Porsche folks knew your Dad.
This site looks interesting, too (although pricey):
http://www.calicenseplates.com/vw-frames-listing.html
Gordon