The Moises app works this way. You buy a song - any song! - from iTunes / Amazon, etc - or rip it from a CD. The song is analyzed by their AI, and then you can manipulate it in ways similar to the Justin app.
Moises is less beginner friendly than the Justin app, and the analysis is not always perfect.
But they have a free subscription tier that is quite capable, so it might be worth a look.