The 300BC used a lot of Fiat Topolino (the original 500) bits and pieces. That radiator with the oddly shaped upper tank is definitely Topolino, and once perched atop the transverse leaf spring front susension, which is also visible just ahead of the engine. The seats also look like Topolino seats. The 1936-55 Topos were the basis for many small Italian cars of the period, from little two seater bodies to drop onto stock Topolino chassis (like our Model T's) to full-out race cars using Topo suspension and larger engines, as with this Siata.
Siata also made a twin cam cylinder head for the 569 cc, 13 hp flathead Topolino engine, boosting the horsepower to 28 with the addition of a second Weber carb.
The very first Cooper Formula 500 cars used Topolino front suspensions at both ends, giving 4 wheel independent suspension.