The Honda GX35 can run in any orientation also but I was looking at the robin myself for the cast iron sleeve. Either will work with a breathing tank mounted anywhere.
The second gearbox is the one I run, 3 to 1 reduction with a dual chain enclosed with grease. The other one seems to be thinner which may help. Welded straight to the sprocket that comes with the box is a 9 tooth gocart primary then down to an 80 tooth mounted to a freewheel. I only run 9 gears at the back so I used a granny 22 tooth and stepped down the crank with a 16 to 40, swapping the cranks around.
Now when I pedal only, the 80 tooth freewheels and when I motor only the 16 tooth freewheels. I can also run both together which works best as the Honda doesnt have much torque and my legs have plenty.
A NuVinci would work great instead of a cassette as you get a greater range and probably better power transmission I suspect. (I have had some trouble with the chain skipping at top speed in 8th and 9th and it isn't quality related.)