I won't fool with incandescent bulbs anymore as they take too much of the little juice provided by the magneto. LED lights are the way to go. Someone in a recent post suggested Walmart had LED bulbs which fit into the old bulb's socket. I think I read that right. I haven't seen them, but if so, then find it in six volts and you're good to go. I use LED lights as I find them and remove the old housings in the bike light and bunch the led's together into a little bundle with vinyl tape, fit it into a cork drilled out to the rights size and get creative one way or another getting them into the old light. Each light is a little different, so you figure it out as you go. The LED lights will have pigtails, so wiring them up is no problem. Remember that they are positive/negative sensitive so if it doesn't work one way, switch the wires around. Actually now I don't fool much with old lights, but make them out of food cans and cover the cans in harness leather. Comes out as a cylinder and is old timey looking.
SB