Wolfgang
I have looked this ship up in the 1990-91 Lloyd's Register - at that time she was owned by Nomura Enterprises Inc, and registered in Panama. The agents for these owners were W. Schoonbeek & Co., Hamburg.
This ship's LR [now IMO] number is 5224247.
Checking Equasis using this IMO number, I find that she is still listed, and is still named 'Allegro'. Her owners are now shown as being Sea Services Transport, Cristobal, Panama.
So it looks as though your father's ship may still be in existence somewhere, but tracing her actual whereabouts / trading area may be rather more difficult.
Regards
David Ford