The javascript code for these examples is in github: https://github.com/OpenHistoricalMap/examples/. A new version of the overlay code is under development but has not yet been uploaded.
In these examples, the variations on the race circuits are drawn in different colors. The Leaflet layer widget allows switching the base layer, and also allows turning the various circuit versions to be turned on and off.
This version can fetch circuit data from both OpenStreetMap and OpenHistoricMap, which allows both historical and current circuits to be displayed in a consistent manner. See Watkins Glen and Thompson for examples of this.