Guido, you may want to give
this one a try.
You only need to:
(1) Open the Pd patch (with Pd-extended, which includes all the required OSC stuff - and best keep the .pd files in the same folder) and select the MIDI output port going to the MotorMix (Preferences > MIDI Settings), or better: add something like "-midioutdev 1" to Pd's startup flags, so you don't have to select it on each launch (Preferences > Startup).
(2) Launch REAPER (4.20+) and set up an OSC control surface to *send* (using default port 9000, you may have to change this both in REAPER and Pd but chances are it works out-of-the-box; receiving data isn't supported/necessary in this patch), using the default OSC config, or the reduced version in the package (which *greatly* reduces the amount of data to only that what is actually used in the patch, but the default config should also work just fine);
(3) There is no step (3). Go mix stuff!
Let me know if it works.