Aprilia37,
Your questions about using various devices off the 6V -12V converter: Phone charger and satnav are drawing only a small current, about 1 to 2 amps. No problem there.
However, a car radio, especially with a cassette player would be drawing more than 5 -7 amps, depending on the size of the speakers.
The generator in the early Aprilia is the same as in the Augusta, the output is 90 Watt only. If you run a car radio you'll use more than half of it already and the voltage of the electrical system will drop to a level which will not charge the battery enough. This in daytime with the headlights off!
A few thoughts about car radio in an vintage car: I only can speak about the Augusta but the Aprilia does not differ much. When driving, there is a fairly high level of ambient noise, like engine and wind, especially when the windows are down.
To hear anything from the speakers, the volume must be quite high. Where to install speakers, hidden? Aerial? It might be necessary to shield the ignition cables and plugs and fit noise suppression to the generator.
I went through all this on my Augusta, but in the end I did remove the car radio. Peace returned.
Further, here all the national FM stations will be switched off by the end of this year, the future will be DAB+
Karl