Open xenia-canary.config.toml (or the GUI). Essential tweaks:

The original Xbox 360 disc for Forza Horizon 1 contains roughly 7.2 GB to 8 GB of data.

If you have acquired a legitimate repack, the installation process generally follows these steps:

The installer was a stark, grey window. When he hit "Decompress," his PC fans surged to a scream. The CPU usage hit 100%. For three hours, the progress bar crawled. Leo watched, mesmerized, as the installer listed files: Skyline_R34.dat, Bass_Arena_Radio.ogg, Colorado_Terrain_Mesh.bin. Finally, a "Play" button appeared.