904 question

I haven't had many 904s cross my bench, but almost all that have, just needed a new frictions, or minor stuff, to freshen them up, and almost always a sprague kit.
Those things are so simple they practically build themselves.
I built my first one in 1976 at age 23. It worked so well, I immediately build three clones.
I still got one left.
They are so simple, that I wouldn't consider installing one even from my own stable, without at least tearing it apart for inspection.