Einstein (https://github.com/pguyot/Einstein) supports iOS and partially supports Android. Watching https://m.youtube.com/watch?v=SB_2DnLVFYI it doesn’t look performant, though.
Here you go: https://github.com/pguyot/Einstein
You need to locate the ROM. The Internet Archive has some: https://archive.org/download/AppleNewtonROMs
Note it is possible to emulate NewtonOS on OS X and, yes, on iOS.