The consensus is to use an MTH tire, from a locomotive with the same-size driving wheels.
MTH has recently announced that they are going out of business, so their replacement tires may become more difficult to obtain. There's another thread on the Forum about this very subject.
You can get tires from MTH or Lionel. They're not THAT expensive, relative to the cost of a locomotive. Perhaps try to guess the size (diameter), order a few from each, and see which ones are the best fit in terms of thickness / depth, and width.
Personally I despise rubber tires, because of their periodic need for replacement and other issues. As long as your grades aren't unrealistically steep, they probably weren't necessary in the first place. But since the loco was made with grooved wheels, now you're stuck (and so is everybody else!)