If you are looking for "free" gasifier plans. The "handbook of biomass gasifier and engine systems" are all you need. This will give all the instruction you need to build an Imbert gasifier and size it properly to what ever engine you plan to run. This handbook along with participation on this forum will ensure you success.
There is nothing wrong with the Imbert gasifier, if its not broke dont fix it. Most that get into this generally want to try and fix it. Myself included :) the reason I think, is one the first things that come to mind is why isnt this already mainstream? It must have flaws so it must need improvement. That is a very true assessment; however, it not in the context we generally are initially thinking. There is actually nothing wrong with the Imbert, the issue is there was missing technology to automate the process that just did not exist until recent times or at least not economically. Automation is an absolute must for any gasifier system in the small scale. With a proper automation system flow issues can be resolved along with electronic air fuel mixing. All this is now well developed from a number of sources. Here is one of those sources. If you are intimidated by this, that is your biggest challenge. Its not the electrical systems that are the challenge its getting over the intimidation factor of it. These automation system can built with off the shelf electronics that are easy to set up and wire. As for the AFR mixer systems, we would be happy to share our code and aid you in building this system.
This here is the Imbert Sizing chart in inch dimensions.