I found one of these recently, although I do remember them when they came out, and I also recall they did have a problem with the rear cylinder overheating.
I intend to get mine going. It is missing the carby and coil, mainly, plus (probably) a heap of smaller accessories. I need to restore (and redesign...) the muffler, since I don't have a carby, "something else" will have to suffice. As for the coil, because of the difficulties these engines experienced with one plug firing and therefore taking all the spark energy, I intend to use two coils, in a "kettering (auto)" arrangement, but triggered by the one set of points. If this works, a small "SLA" 12volt battery will provide enough power for the average mowing session.
I have a small Mikuni carb, hopefully this will pass enough mixture to get the thing going (certainly to see if the project is worth while). The exhaust I will redesign, but because both cylinders fire at the same time, there's no point in designing "extractors" as such. A decent tuned length system should do.
I was talking to a proffessional mower repairer recently about my plans: he was very helpful (he told me about the plug problem, and how they quite often, run on one cylinder etc) and I commented that the engine didn't seem worn at all. His explanation was that nobody got them going lomg enough to wear them out!
Jeff.
PS: pnce running properly, they must have been very smooth, with both cylinders firing in opposition, there would be no vibration.
