Hubert,
you can also find information on the forum because some people have built similar ones. The keyword in the search can be Homebrew sbitx or whatever you are looking for. The RPi must communicate with the Si5351 and WM8731, otherwise it will not load the application. Look for the problem here: the Arduino has an I2C address search. I tried reading some threads about this and several people are using the Mikroe WM module.
--
Gyula HA3HZ
HA3HZ