Headlamp: The best headlamp for the conversion is a 7", side mount headlamp with an H4 bulb. Delkevic and others make a knockoff of an S650N headlight that works quite well.
When using a headlamp(like the one shown above), you will need to purchase or construct a set of headlamp brackets. A common approach is to use fork-mounted headlamp brackets.
Fork mounted headlamp brackets: You will need brackets which will mount on a 38mm fork and that are at least 4" in length (5" or greater is recommended). EMGO makes an aluminum set that looks good and is relatively inexpensive. EMGO p/n 66-35832.
Alternatively, you can purchase a set of headlamp brackets specifically designed for the Seca II for around $40 USD:
