Ways to get fusion parts
First, acquire some bronze keys in live racing. Second, accumulate the free bronze crate Donna gives you every four hours. Next, get a part from acquirening a Daily Battle. Also, race every cup & trial race you’ll be able to to acquire fusion parts &/or keys. Lastly, buy cars and strip them for fusion parts (for example, you’ll be able to buy more t1 Fiesta STs to strip to get fusion parts for your t3 Mustangs or buy more t2 m235i to strip for your t3 BMW M4s). You will not get Mustang or M4 stage 6 fusion parts this way, but rather common, rare, and epic parts which are usable on any Ford or BMW cars in the online game (Ford for Ford and BMW for BMW). To get stage 6 parts for the Mustang or M4, you would have to strip Mustangs and M4s.
If you truly need gold, use csr 2 cheats.