This might be a lot more complicated, bulky, and expensive than it might seem. Are you going for and/or okay with the Mad Max aesthetic?
Do you have access to fabrication equipment and a lot of spare parts? Generally, switching between voltages gets kind of goofy; e.g. I had a friend that INSISTED on building a home stereo from salvaged car parts, but it ended up being kind of clunky, as his cabinet had to have the equipment on the back to step the voltage down to the appropriate levels. It worked, and it was sort of cheap...
I'm thinking, to start, you'd probably need to figure out how it was run in the car. I've seen a decent variance in how headlights are run in cars. Generally, depending on the voltage it was run at, there's probably something that was stepping the voltage up or down, since cars run 12-14.4V. Then, you'd likely need to figure out how much of the headlight housing is needed (lots of times, without the housing to focus the beams, the patterns from headlights look terrible). That would give you a starting point of the physical size, and then the voltages you'd be needing.
And you know they're LEDs, and not HIDs, right? IIRC, the HIDs use a ballast that arc light at a pretty high voltage, and people have accidentally electrocuted themselves messing with that, just as a safety FYI, haha. I remember back in the days when HIDs were new, some people messing with retrofitting those and ending up injured, haha (I know, I shouldn't laugh at something like that).
If you're lucky, these will be able to run off some kind of voltage that works well with Li-ion batteries, so you could build a battery pack. You may end up needing to get the right equipment to step/up down that voltage, assuming you want to run this on Li-ion cells.
Fundamentally, I think the issue you're going to maybe run into is a headlight is generally going to probably not be ideal to run off a battery pack that would be ideal to run on a bike. Headlights are used to having access to dozens of amps on tap from an alternator, powered by a big ol' engine, and gallons of fuel.
It might honestly just be cheaper, easier, more effective, and more convenient to get a good headlamp for yourself when biking.
I ride a 29'er plus bike, and I can't think of a way to rig something up like this that wouldn't be a bulky pain...
If this is all for poops and chuckles, though, then that's different!
