There are various mechanisms that can cause the top of the input lever of the 1939 Gravity Power machine by William F. Skinner to move in an elliptical orbit.
I already put details on how William Skinner was doing it himself in my video 1939 Gravity Power Reverse Engineering Details FULL VERSION
A simplified method that I came up with is by using a tilted wheel with a connection offset from center. Although from it’s own plane, it is a full circle, on the horizontal plane, it will trace an ellipse.
Here is what I came up with…