Was any mod done to a C47 when putting it on a civvy registration, or was the change just a paperwork exercise? I find it hard to imagine that beyond a radio fit anything much was needed.
The vast majority of C-47 were built as cargo aircraft, many others only had bench seating on the sides of the cabin for paratroopers. So a big part of the conversion of the standard C-47 to passenger use was the removal of the cargo flooring and installing seat tracks, for both airlines use and corporate use. This also required the installation of passenger reading lights and fresh air vents.
Also many of the C-47s had cabin windows with plugs in the middle of the windows that could be pushed out allowing the crew/passengers, if armed, to fire at enemy fighters if attacked*, these had to be replaced with standard cabin windows.
One of the DC-3s that I flew was actually a C-53, this was a C-47 built to carry paratroopers and one of the differences between the C-47 and the C-53 was that the C-53 carried more fuel than the standard C-47.
* I never heard if a C-47 was ever defended in this manner or not.