a quick search pulls up images pretty quickly. I don't think I've ever seen one of those.. sort of looks like an old Union dynamo headlight attached to a battery case.

The hard part about converting some of these lights is that you have to add heatsinking for the LED , plus the optics aren't a good match for the radiation pattern of a LED. My first LED conversion was an old Union dynamo light, and it ended up that I only kept the reflector. I was using a Luxeon V side emitting LED, which was somewhat unique in that most of its light went out to the sides where it could be focused by the reflector. The beam pattern was pretty poor. A lot of light still went out the front of the LED, I think.

I've had much, much better results with some of the molded optics, especially those from Ledil. My Cateye Micro II conversion is an example.

I've seen reports of conversions of "classic" headlights where the LED was simply placed in the location where the bulb had been. They did look nice, but I have doubts that the beam was equivalent to what can be obtained with molded optics. The problem of heatsinking the LED is tough to solve too. The best case scenarios are to add some venting to the case to direct air to an internal heatsink, or try to create a thermal path from the LED to the steel case, or to try to create a thermal path from the LED to an aluminum handlebar or mounting bracket that can serve as a heatsink.