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