I've done similar inter-language changes, and I have always found it easier to not change the language of the business logic. (Unless the change gives you something really big - for me I often port stuff to numpy because I need the vectorized code, but that's only for very specific problems).
If I had this task, the place where my brain would go would be to find a way to compile JS into C and then use C calling conventions to call the functions from Python. Keep the JS code around so that if you need to change anything, you change it in JS and then recompile to C.
I don't know the JS space very well, but can you get a JS interpreter that lives in Python? That way you can call JS functions from Python?
I don't like transpiling, there's always enough differences between the languages that something bad happens. When I've run into issues like this, since I'm an "old guy", I tend to try to get everything into C calling conventions and use that as my base interface.
Worst case, there has to be some good JS interpreter that can give you a C interface that you could call from Python. So you'd have Python -> C -> JS and your business logic can still live in JS (if your port is because of efficiency and you need compiled code, then you can ignore me.)