I would be looking to use LED style lights instead and a bicycle
generator with maybe a solar panel too. I also think that using
automotive stuff would weight too much and have too much drag. Most
modern alternators create way too much current for just lights
anyway. Even an older 40A alternator wouldn't be as good as say going
to Nashbar's or Performance's website and using bicycle stuff
instead. Maybe if these Amish had electrical rear defoggers that they
were using then you might be able to justify something like an
automotive alternator.

To the person who said something about why are they even using
electricity in the first place, they aren't always necessarily against
it. It's more of a self sufficiency type of thing (meaning that in
some Amish communities it's OK to use electricity for certain things
as long as you generate it yourself).