The Lionel parts site shows the O ring out of stock but it says to use part #6208063615 in its place. That O ring is in stock and is $1.50. Interestingly the substitute part is a 1-1/8" traction tire.
To find the parts and manuals I just click on the Support button on the Lionel homepage, the parts site comes up, then enter the 6 digit SKU in the search field.
Rogerdodger, thanks for getting this on the right path.