You can still download PPaint 7.1c for free from Cloanto:
https://www.amigaforever.com/classic/download/
What exactly is the problem with saving icons under different icon.libraries?
Do you know that OS 3.5/3.9 has a different newicon.library than those in the original NewIcons 4.6 package? The emulated newicon.library from Stephan Rupprecht, which is also included in OS 3.5+ converts ColorIcons into NewIcons to make them available to old programs like Iconian etc. (I think this version of newicon.library is also installed in ClassicWB, since I once did that for ADVSP)
NewIconEmu: =>
https://www.stephan-rupprecht.de/