I don't have any experience with wood gasification, all I know is when you heat up wood, but don't let it combust, it gives off hydrogen, which has a wide flammability range...very easy to get it to burn in any engine it's routed to.
Here's my "go to" website for converting lawn mower engines and car alternators to a 12V generator (google has many more):
http://www.qsl.net/ns8o/Induction_Generator.html
Steam from boiling water can produce high pressures. Still used today. At otherpower.com, they used DIY wind-gens and solar-PV panels, but sometimes the wind and sun are weak, plus they had lots of wood. They found an old steam-set that was lawn art. Steam engines are heavy-duty, so it was not hard to get it running again.
Steam gen from otherpower.com
http://otherpower.com/steamengine.html
Depending on your skill-set and tools, I would prefer building from fairly new scrap, an Organic-Rankine-Cycle (ORC). Think of it as "Freon steam". Can use propane, butane, freon, and toluene as the liquid/gas conversion media. (of course, Butane and propane are flammable). It can be heated from the sun when its out, or fireplace chimney heat when the sun is not.
I love Stirling engines, and yet...they have limits. They are not power-dense, meaning you need a large engine to make small power. That being said, they operate off of a small difference in heat, resulting in a small difference in pressure. They can run off of solar power and air-fan cooling.
Here's my favorite, called a 6-cylinder when searching Google. It actually has 3 power cylinders to be the minimum for self-starting (6 heat exchange cylinders). It's a 3-cyl Gamma. If you took a stepper motor the size of a coffee cup (salvaged from a large office copier or ebay), this could charge a 12V battery to supply cell-phone, laptop, rechargeable flashlight cells, etc. The first 3 minutes are the details of construction, I linked the ten second "money shot" at the end.
http://www.youtube.com/watch?v=lvvyjJ8wz3Y#t=3m50