Replacement treads are available for $10.24, which includes parts and shipping.
Note: 11120 is the model number for the current version of the 2-in-1. There have been many versions of this including models 4-D, 4-E, 4-G, 4-MB and many others dating back as early as the 1940's. Replacement treads are slightly smaller than those of the original units. Measuring from center at the largest point, the small ones are 5-3/4” X 8-15/16”. The large ones are 5-7/8” X 9-13/32”. The set includes one of each.
Below are the application instructions, which are included with the treads.
Additional replacement parts available include hardware for the chair and step, and legs with foot caps. Based on the size of the tread, this would not be suitable for other vintage models.