...just an update. apologies to anyone who tried the bulb above. I didn't realize how bad it was until I used an improvement. It is Home Depot sku 1002 631 975.
620 lumens, 7.5watts, 3000K color. 35 degree spread. 9 bucks each.
The spread is important. A full 180 degree spread puts almost nothing down the road to see with. 35 degrees is quite nice with 620 lumens. My Newport stock magneto runs two bulbs just perfectly. At idle it dims a bit and has very slight blinking. The MR16 bulbs have all the internal circuitry necessary to take the ragged 3 to 18 volts ac coming straight off of the Magneto and smooth, rectify, and regulate it for the LED inside. I've run these for six months of daily use and they are appear to me to be as healthy as when I installed them.
Next spring/summer, unless something better is on the market, an updated version of these from Philips that is 720 lumens, 7.5 watts, 3300K (slightly whiter which would be nice) and 25 degrees of spread to put more light down the road.