The old Brit bikes had a huge zener diode in a heat sink to ground the excess output. Today there are more efficient ways to regulate the voltage output of PM alternators and they are replacing the old excited field alternators rapidly. My current motorcycle has a PM alternator. Nice thing about a PM alternator is that it is actually possible to push start a bike with a totally dead battery. You’ll need a pretty long hill though. Especially if the fuel system is EFI but it CAN be done.