I have a 55/60 watt Sylvania sealed beam in my bike. It has never been a problem. If you were to have a problem, it's probably the stock battery.
My charging system puts out about 13.6 volts at idle with or without the light on. It goes up over 14V as the revs go up. It is plenty adequate for the system.
If the headlight assy will fit a G5, it will fit a C5, it's the same casquette.
Bare