Candidate motor as a generator. Some testing...

[loquin]

There are two approaches you could take with the above generator. If you decide to go with a 6V approach, the generator will provide you with enough voltage, even when the motor is at an idle, to power 6V lights & other equipment. This also means that you could use a the simpler step-down (buck) switching regulator.

If you want to go with a 12V power system, there are advantages. 12V lighting, even 12V LED lighting, is available. In addition, accessories like brake/tail/turn signal lighting is readily available. So, you wouldn't need to customize or build from scratch.

However, the generator won't provide enough voltage to maintain a charge on the battery, until the generator RPMs get over 2200 or so. (Unless you create a boost/buck regulator, that is, which will boost the output voltage when the input voltage is low, as well as trim the output voltage when the input voltage is high. But, a Boost/Buck regulator is a fairly complex design...)

So, your choices with the 12V approach are to accept that you'll be running off the battery when the engine/generator RPMs are low, and only be charging the battery when the RPMs are up above 2200 or so, or use a complex boost/buck regulator.

Note, that, IF you're using a lower RPM engine (Like the harbor freight 79cc motor,) if you have to drive the generator after the clutch, it might not make sense to go with the low voltage system, or with a buck/boost regulator, as the input voltage would always be either off (as the generator would not be driven at low RPMs, or the voltage would be above 12 V anyway.

 

[AussieSteve]

Even when driving the generator after the clutch, a buck/boost reg is useful. It allows you to gear the generator lower to avoid excessive revs when the engine is revving high.
ie If a 2500rpm genny is geared to do 2500 when the engine is doing 3000, then when the engine hits 8000rpm, the generator will hit almost 6700rpm.
If, instead, the generator is geared to do 2500 when the engine is doing 4500rpm, it will spin at 4400 when the engine hits 8000. Much better, but a lower output. That's where the buck/boost is useful.
They're a little more complex, but not that much. A decent buck/boost can be implemented with only one inductor, but 4 switches, (MOSFETS).
I'm probably going that way with my setup. It's looking pretty good so far.