Rules? Fit & function.
If something works, leave it alone.
If not repair or replace, sometimes with a new generation of hardware sometimes not.
My '74 sports a '50's IGH/Hybrid drive-train, my '77 converted to an even more obsolete FG drive-train, my '92 to a then "modern" 10 cog drive-train.
None will be featured in the Period Correct Police 2019 wall calendar, which suits me just fine.
-Bandera