WMO is a short stroke engine base on crank case as well as internals couple years ago. I think WMO stands for World Model Edition or something. From memory they comes with a plastic carb also. It runs ok compares to a RX21. Not as fuel efficent as the new crank case. Just make sure you change the rod often, I think they are not as well manufacture as the R4. Understandable when you consider the price.

As far as Rex plugs goes. They are exactly the same as the Nova Plug... Different packaging!