Hi Mickey,
Like you, I down sized, and decided to build my next RR inside and on benchwork so I would not have to do the hands and knees grubbing in the dirt. Most of my problems over the years have been with drive trains, rather than the detail parts, probably due the the fact that I put a lot of hours on them.
I bought two Connies when they first came out and a few years ago one of them failed due to the gear problem. I decided to buy two of the brass gears on the axle, fixed the broken one, and put the other inside the tender of the one with the original gear.
https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=69_142&products_id=10285.
My shays have held up so far, but I did replace the gear on my Spectrum Mogul, I believe I got the gears from NWSL which is going out of business but is still taking orders till supplies run out or til they close at the end of July(i think). The same gear works on either the 4-4-0 or the spectrum Mogul. Some years ago I did some "How To" repair instructions which are now in the FAQ section of the forum:
https://www.bachmanntrains.com/home-usa/board/index.php/board,10.0.html.
As for the Climax and Heislers, my suggestion is to go ahead and get what you can in the way of drive train parts while they are still available. I base this on the probability that in retirement you may run more and thus have more failures. This is true actually for all your Spectrums and non-Spectrums as well. My overall thinking is that I want my locos to run even if they are missing a few parts. Some parts can be fabricated from styrene plastic. Being somewhat of a scavenger, I have also bought junkers on Ebay or at Swap meets, but only if the price is very right.
As for an failed locomotive it is sometimes fun to remove the gears and make it into a dummy, and use it for double heading, or as decoration in your roundhouse or shop building. I have also used parts scavenged from a different loco to replace a similar part, like a bell from an Annie to replace one on a Shay or 4-4-0. I also use epoxy (JB Weld) to replace parts that have fallen off. When a part falls off I always put it in a a snack bag and label it til I can do the repair.
Anyway most importantly congrats on your retirement!
Have fun with your trains!!!!
Bill