You aren't ruining the economy (there is no economy). You aren't griefing online (lobbies are empty). You are simply unlocking the full potential of a 13-year-old masterpiece to finally drive that Pagani Zonda Cinque Roadster around the Red Rocks Amphitheatre.
Depending on your platform, your save data is handled differently: forza horizon 1 save game hot
Always create a duplicate of your original save file before attempting any modification, as data corruption is common. You aren't ruining the economy (there is no economy)
Plug the USB into a PC and use the Horizon tool to open the save file. forza horizon 1 save game hot
You can find pre-completed save files on community forums like Xpgamesaves The "Rehash and Resign" Step: