From the extracted source ZIP, activate Emscripten 4.0.23 and run:

  cd emulator/wasm-genplus
  emcmake cmake -S . -B build -DCMAKE_POLICY_VERSION_MINIMUM=3.5
  emmake make -C build -j2

The generated core is in src/main/js/. Compare both files with runtime/core/ using SOURCE-INFO.json. The source ZIP also contains the frontend, approved cartridge source and notices.