Thanks to dcuny for finding Spleeter

Watch Video

Reaper Scripts
ReaTrak spleeter audio item to 2 stems.lua
ReaTrak spleeter audio item to 4 stems.lua
ReaTrak spleeter audio item to 5 stems.lua

This works on Win, and should work on Mac but needs a bit of messing around because every version is different, here's a Mac video


Install Anaconda Python using defaults Win (C:\ProgramData) set for all users if you like.
Unzip and rename folder to spleeter
Copy the spleeter folder to your Reaper Scripts folder or Scripts\ReaTrak\
Open Anaconda Prompt (search for it with Windows Search)
At the command change the directory to the spleeter folder location by

cd C:\Users\User\AppData\Roaming\REAPER\Scripts\spleeter

install using conda with this command

conda install -c conda-forge spleeter

It will take a while, when it ask to update packages select y (yes)

Once done you can insert a song into Reaper select the audio item and run a 2 4 or 5 stems script
the first time will download the package for 2stems 4stems or 5stems

When done the stem tracks will be inserted into Reaper,
they will be in a folder spleeter_output in the same folder as the source media

if you get an error Intel MKL FATAL ERROR: Cannot load mkl_intel_thread.dll.
remove or rename 'System32\libiomp5md.dll' and 'SysWOW64\libiomp5md.dll'

You can get the tempo map from the original item or the drums stem with
Script: ReaTrak get tempo map from item.lua (needs