I assume you do not want to do this work yourself. The 2-4-2 is a very easy loco to disassemble.
You don't explain in what way the 2-4-2 is "showing its age." Generally these older locos do not run quite as smoothly at low speeds, or as quietly, as new can-motor locos do, but then they didn't when new. Have you tried just dusting it off, lubing it well, and particularly cleaning its wheels and center contacts to get better contact, etc.? Often that "tunes up" an old loco that has not been abused quite a bit?
If it really needs attention, I'd start by asking your nearest Lionel dealer. They usually know. And any competent repair person will honor your request to leave the scratches and dings there - and make sure not to add any more.