Barring the ton of licensing stupidity ongoing (maybe they'll fix it, who knows), as far as I know you could actually do that by forking the repo and then connecting your fork to bountysource. Open an issue for each target OS, set the amounts, and then publicize that fact on as many social media channels as you have access to
However, I wanted to draw your attention to "are you asking the right question" because there's a very real chance that the things you actually love about Winamp would not carry over to any such ports, since it's not "recompile and get the Winamp on Windows experience" - they would be Gnome and macOS apps, behaving similar to other such apps
This is more significant (to me) than the iPhone 16 launch, I'll pay for people who can port this to Linux (Gnome) and Mac OS.