Sorry for the grave digging but this is such an interesting topic I thought I would share what I've learnt.
I've thought about a similar thing myself, I thought to myself of using LPG/Butane instead of Petrol primarily for environmental reasons but also for reliability reasons.
Please also use LPG. Save the earth and all that.
Forget using a car alternator, they're built for a single purpose. ANY project involving this WILL require using a direct drive washing machine generator head.
Forget using pulleys also. Its too costly to go down this route and finding an A pulley to fit a victa requires butchering one from a self propelled model and with very little guaranteed success.
Look familiar? It looks like the motor from a modern washing machine.
This is what I mean:
Using this project combined with a programmable charge controller you'll be able to charge a battery bank of almost any size. I would recommend using a set of multiple windings at different voltages so you can charge a 12/24/48v battery bank.
It will require significant machining work but its not anything too difficult.
