Be sure to copy the folders "DATA" and "GFX" to the sub folder "/Contents/MacOS/share/s25rttr/S2/" of the application package.
From the existing Intel version of the s2client, copy the file "s2update" from "/Contents/MacOS" to the same folder of the Apple Silicon version.
When starting the Apple Silicon version, macoS tells about a "damaged" application. Go to System Preferences, "Data Protection & Security", scroll down to the note "application has been blocked", and click "Run anyway".