Hi Jim,I am glad you got your Marshall running. Interesting old tractors aren't they? I remember my neighbour using one of those right up into the early 60's to haul / power a threshing set , i.e. drum + a baler . When he was travelling down the road you could hear it for miles, "thud-thud-thud- thud!"
I can't help you much about the Wartime gas producers, sorry. I know in France they called them "Gasogene" tractors, The idea was used for cars and trucks too. There was a BBC TV documentary series programme called "Wartime Farm" and in one episode they actually re-created one, using an old 1930's ambulance