That makes sense, your best bet to make usable power would be an alternator. You could reuse the pulley system found in the car to make the physical hookup easy. There’s two things to regulate power - the voltage regulator you mentioned is the first. Many alternators have a built in voltage regulator. A particular example is the '63 - '83 GM alternator that looks like this that has a simple 1 wire output that’s regulated. I know some newer GM models have a regulator built in, some don’t. You’ll have to look up those and other makes on a car by car basis. The next thing for regulating voltage is a battery. A lead acid battery (car battery is this type) wired together with the alternator will smooth out the spikes and the dips in the voltage generated, but isn’t required. If you have lights for example, without a battery, they’d likely flicker or brighten/dim when the alternator load or RPM shifted. More sensitive electronics might not like the voltage changes. Once you got the alternator running then you’ll be able to use any car accessories like headlights, 12v USB chargers, stereo, etc. I found an informative forum post here about the problem as well.

https://mechanics.stackexchange.com/questions/88789/trying-to-use-a-car-alternator-for-a-power-source-not-for-car