First, welcome to the Forums.
If the battery isn't holding a charge, may have a bad battery, even if its new. Any Autoparts shop should have give a free testing.
If the battery is good, something might be grounded out, and killing it. Go over all wires, and check the ground clusters. They cheaply done.
Still dead, replace the Voltage Regulator. It is a small metal box attached to the triple tree, behind the headlight assembly, with a small cluster clip. Make sure it is bolted to the frame, it needs that ground point.
Finally, if needed, replace the Magneto..
That would be the order of operations I would take.