It seems there will be some changes needed to port to Qt6 for qml-box2d.
The original version is no more maintained (https://github.com/qml-box2d/qml-box2d), but some work has been done in https://github.com/mleutelt/qml-box2d, https://github.com/MisterGC/qml-box2d or https://github.com/FrankMamuda/qml-box2d for Qt6.

Maybe out of scope but would be nice to see if it can update Box2d to 2.4.1 (https://github.com/erincatto/box2d) if it is interesting.