antique cycle 6v adding led headlamp

IMO that voltage reg may have too low an upper voltage but its current capability is good. Otherwise maybe units like Auto DC 3-35 to 1.2-30V Boost Buck Converter 12V 24V solar voltage regulator charger circuit (2A max) or others based on the LM2576 etc. The XM-L2 LEDs have viewing angles of 120° which IMO is way too wide tho I haven't analysed the spread. The cap size is based on the lowest generating frequency (times 2 assuming a single-phase "alternator" fully rectified - ie, the period between 2 fully rectified peaks) and then whatever voltage sag is permissible (ie, total current and minimum operational voltage). But the dc-dc converter should look after any ripple in the rectified supply provided it's above the converter's minimum operating voltage.

Hope you're sitting down. Here comes another silly question. Actually I think that's why I'm coming off a bit of a blowhard here, since I really don't know a damn thing about what I'm talking about. IMO that voltage reg may have too low an upper voltage but its current capability is good. Again, surprisingly difficult to web search optimal arrangements for two or more LEDs. I was working under the impression that wired in parallel, three 2.9v LEDs would still draw 2.9v. That's why I anticipated no problem with that regulator. It's suspiciously tiny by the way... about 1cm x 1.5cm x .5cm. If it's not too complicated, can you explain what you, and a few others, seem to be suggesting to regulate the CURRENT fed to modern high powered LED's rather than the voltage?

Well, you would not have parallel LEDs if (limited) power is an issue. You would series them into the highest series voltage string you could have to suit the supply voltage. If you used switching current limiters on both or for all, that's different, but I'm assuming a single supply voltage (eg, 12V) with analog current limiters (resistors etc). I'll simply state that LEDs are current devices. Their output is not regulated by voltage, but current. And max current is the critical parameter wrt LED life. (Their voltages vary even off the same production die.) And BTW - use Schottky diodes for rectification. They'll have under half the voltage drop of regular silicon diodes.