Did you see this sentence in the link you provided?
"However, as to mass use, that seems doubtful. There is simply not enough of a supply of chicken manure to provide fuel for autos on a mass basis."
Before embarking on designing and building something you should find out how much energy can be obtained in the form of methane from a given amount of organic material. Methane is often collected as a byproduct of treating sewage and you may be able to Google and find some numbers for the Methane produced from a given quantity waste.
An open mind is a virtue...so long as all the common sense has not leaked out.