There's a website which lists a FEMA report on using wood gas, including a 'how-to' on building a wood gas generator. It will supposedly run any gasoline engine; there's a picture of a tractor with a gas generator strapped to the side, and the farmer looks to be merrily plowing away.
Look up wood gas generator on google, and you'll find a ton of data on them. I even once found a picture of a car with a wood-gas generator in a trailer hooked up to the car! If we got into a real pickle for gas / oil, I think wood gas and biodiesel would be the way to go.