I added lights to my first TL (a '73; franken-bike was my name for it because it rose from two dead bikes) by first studying the existing magneto coil, trying to guess how many wraps it had, etc. That gave me some idea, after using the voltmeter, how many more windings would be likely to create about 13v. There was already an unused stator, so all that was needed was to wind it for headlight use. A trip to the electronics supply, and an hour or so winding the appropriate magnet wire around the unused stator gave me the power source I needed. A subsequent trip to the motorcycle scrapyard provided the donor lamp and bracketry. It actually worked OK without a voltage regulator, but after a number of months, the bulb did eventually burn out, so a voltage regulator would seem to be the wise route to take.
Obviously had too much spare time back then while in school.