The stock headlight is sealed, so you need to hack/mod it to replace the bulb.
The most common aftermarket headlight and the brake/tail fixture both use BA15S bulbs. You’d want a 12v 21w “larger size” bulb or you can mess with the double filament one if you use a high-beam switch. The brake and tail lights are the “small size” bulbs. You’d want them in 12v 10w and 12v 5w.
BUT!!! If you run that 12v lighting coil you can plug these bulbs
into all three slots and run them just fine. I use a regulator for the headlight but not for the brake light, and I still use my old 6v coil for the tail light. I don’t know how or why it works but it does. (edited)