Lost post
I will try to recoup.
This will give the basics. The article on reviving old trains.
http://www.trains.com/ctt/default.aspx?c=ss&id=22
Good working engine. I like sound and a low throttle startup. The motor should not overheat. The axle bushings should be good with little play. The armature plate should be flat. Grooves indicate a lot of wear. Poor connections on the e unit won't help. 40 year old brushes and springs could be replaced.
You can get reconditioned motors, ensure they are reputable. Or by one to compare to the old an extra wouldn't hurt.
Two methods for testing are by comparison with another engine and pullling power.
I like the 2025. Don't have one but just have a 2026 and clones.
For performance run the engine. if the rims shine that is good, cycling the e unit will help connections. Sitting idle on a shelf is for trophies.
Good Luck
Bob