Not sure if this applies, but:
I'm one of those who managed to hang on to his first O-Gauge set, the 6-1661 'Rock island Line', of 1976 vintage.
Over the years, I ran that 8601 loco, an 0-4-0 with no real prototype, so much that I actually wore out a set of brushes and a pinion gear (both replaced) and swapped out the original sliding contacts with roller ones.
The part that wasn't so easily replaced was the driver gears--they're so worn out that the teeth have grooves in them and idler gear between them started slipping the last time I ran the set maybe a year or so ago (I posted a photo of it on a 'your first train' thread). Ironic that the plastic gears are still in good condition, but the metal ones wore out.
During the course of looking for replacement wheels, since I couldn't bear the idea of permanently retiring my first engine (despite all the other motive power I now possess) I managed to acquire a number of almost-complete-but not-quite-the same versions of that loco, among them being a near-mint example of the chintziest steamer MPC ever made--the infamous 'Working on the Railroad' set loco with plastic drivers , a Black Cave Flyer set missing a boiler front and one side of its siderod guide, a shell from an 8803... and a duplicate of my original 8601.
I was on the horns of a dilemma. I wanted to just swap out the drivers (I found a pair of postwar drivers and a motor unit with drivers) and simply paint them red to match the originals--then I started running across and acquiring similar locomotives that could be more suitable parts donors:
-- I could have just swapped in the postwar motor and gained an E-unit (the loco had an open-frame AC motor, but manual reverse), but that left me feeling that this wouldn't be the engine I grew up with.
--I could have used the duplicate 8601 unit and simply swap out the broken parts--the shell was certainly in better condition than the original (I had broken the screw shaftin the body shell, but eventually built up a styrene box wedged and glued into the space so it could be screwed to the motor frame), but again, this wouldn't be the engine I grew up with.
--I could have used the drivers off the duplicate 8601, but they looked like aluminum wheels with a red plastic insert for the spokes. My original was more like postwar drivers with red-painted centers (with a chip on one proving that they were painted). Puzzling that the two seemed to have different drivers, but I don't know how long that set was in production. They just didn't look right.
--The Black Cave Flyer loco was to be the original parts donor, but I passed on that since the loco came with its tender and the rest of the cars from the set (you can see them being "transported" in my Williams NW2 video ). I then I decided to restore that set since I remember wanting that set way back then (1982?) too--it was the only other example of "my" loco I ever saw in the Lionel catalog.
So I'm back to my original plan of having a service center replace the original drivers with the postwar ones I bought, then I'll polish the rims and paint the centers to match the original. It's definitely not cost-effective, indeed the labor is pretty much guaranteed to cost much more than the engine is worth, but it'll still "be" the engine I grew up with.