Put a multimeter (on a 10A scale) in series (ie between) the battery +ve post and it’s lead. With engine off, you should see -ve amps with electrics on (eg brake light), and with engine running a charge (+ amps).
Multimeters can be cheaply found eBay etc, be sure to get one with a 10A scale.
You’ll have a better idea of how to proceed once known, and said multimeter can be further used to troubleshoot wire continuities etc.
I assume the bike has a magneto? Just sold a Fizzy with magneto / 6v system… you almost had to choose between head, brake… indicators as having all on together was a big ask 😂.