Some of the old generators from the old cars have been used for kids motor toys. When you put juice to them they are a motor, without juice they are a generator.
I have a Riding mower that was built with a belt drive generator of this type. It's the starter and when the engine is running it is the generator. Factory made that way. Unfortunatly that engine is junk or it would be a gennie for me.
I don't know the power or torque on those generators, but they move several kids around as a yard toy, and at 12V you could use one at each wheel for 4 wheel drive 
If 2hp, 4 would give you 8hp. And regenerative braking is kinda built in too.
It was interesting to see the numbers posted for starters. Many electric cars and kits seem to be using 20hp motors, these are for road use too. Instead of one large motor I would rather have 2 or 4 smaller ones myself. SHould be easy to make end caps for starters to alough airflow and add a fan like an altenater has. Would help cool it some, but would it be enough? As for starters, yes they do have alot of power, I have had vehicle problems several times with a manual tranny and walked the vehicle off the road running on the starter. It was slow, but worked as long as the battery was strong enough, and that's cranking an engine and alot of power loss with the tranny and other stuff too.
Using 2 or more starters would reduce the load that each has to carry on it's own as well as upping the HP available. And it would all be 12v too! You could try upping it to 24V, that might work or it might not. I have ran 6V starters from the old cars at 12V just fine myself. They actually worked better, not sure about the 12V since i have not done that yet.
