I have a late JT, it does have the original Engine and it is a fascinating engine to say the least.
I asked several people why did MM put a third party engine into the first really designed tractor after the merger of 1929. Twin City had a reputation of very robust engines, and the Waukesha is not very robust.
So the answers I did get back from those I asked was the market kinda of dictated the move to a third party engine. Also several tractors of this era also used third party engines, Case, IH and many more.
The best answer I received was since the whole market at the time was to build a row crop tractor aimed for the small farmer one or 2 bottom plow market. Examples IH the powerhouse of mass production had the farmall line, and John Deere with its letter series
Well the engine they chose and had a lot of the design work with, kind of put MM behind the eight ball right from the start. My JT engine has a lot of stress cracks in the block, lots of bubble gum welds here and there and not to mention the manifold was not honey either.
I contacted the Waukesha Engine Historical Society and asked about this engine, like every one said, there is very very little information out there on this engine. Dennis from WEHS told me according to records from 1932 to 1937 about 7000 engines were built and after the run was done all the tooling, patterns and plans were either returned to MM or destroyed. I did get a manual for the engine from him for a Waukesha Model 4-55 engine which appears to be the same engine as a JE
The engine is not very heavy duty, I can see why it was a candidate for replacement.
I am planning on making a new manifolds, I am currently working with a local foundry on getting it right. I think it will be the fall of 2018 when I will have the manifolds cast. If anyone is interested, drop me a line.