Lionel dropped large-scale Thomas many years ago. Parts are difficult or impossible to find; I've tried. You might be better off buying a Bachmann Thomas, as I did. It's a better mechanism, although it does have quirks of its own.
Given enough info about the motor (dimensions, rated voltage etc.) it's probably possible to find a close match. I would expect Lionel used a commonly available part. Ebay has loads of motors for toys, RC cars etc.
Have you removed the motor and tested it - proved it has failed?