You can use whatever decals you want, both options are corect. Back in the late '70s, Fiorelli and Coppi bikes were practically the same thing. Starting from the '60s, Fiorelli brothers linked their name to Fausto Coppi's, maybe to increase the sales, but the bikes were still made by them, so it's nothing wrong in using Fiorelli decals, if you prefere them. The letter "C", engraved in the fork and under the bottom bracket stands for "Campionissimo", wich was their top model at that time. I am restoring one and i will use Fiorelli decals, even though this brand is far less popular than Coppi. If you want to sell the bike, make it a Coppi. If you want to keep it, there's nothing wrong making it a Fiorelli.