When they wear out?

This would require you have the Equipment and Skills to machine metal to tight tolerances. Best Option, Find replacement parts at a show where you can examine them in detail.

You might try what I did with my Dad's mid-'30s wind up Marx loco and train.  It did still run, but it had much the same qualitative problems your loco has.  I decided to leave it alone and retire it to a place of honor (photo below).  I shopped swap meets and found exact replicas of the loco and cars and bought several that, while just as old and sometimes in need of significant work before they would run, looked to have far less worn bearings and motor area, etc. I stress swap meets here rather than e-bay, because you can hold it in your hand and fiddle axles and all to determine the exact state of wear in the parts that are important to you.  

I then assembled a "new" replacement train that was identical to my Dad's, which is the only one that I run now (not that I run wind up locos more than a few times a year) and have retired my Dad's loco to display only (below).

Interestingly, the replacement B&O Marx boxcar cost more than everything else in the train added together - apparently it is quite rare, and not only took a long time to find but cost, I think, around $300.

This is me with the original set in 1953, which was durable enough that a four year old could not damage it but probably well worn by then.