Firstfives.org

Would the ignition system benefit from a new coil after 40 years or does the coil not degrade over time?

Pull a wire off and look at the spark, if it's good, so is the coil. They can get weak, but usually if it runs well, the coil is fine. Absent of problems, I wouldn't bother replacing it.

Wouldn't hurt to buy a spare and keep on the shelf. Makes a great diagnostic tool, as its quick and easy to replace.

If you really want to buy one....

I've never solved a problem replacing a coil, although I have a dozen spares collected over the years. Maybe someone can tell me what makes one different from another other than the part number and some need ballast resistors.

Some prefer the blue vs. black

What makes them different? For one, not all spark plug cables have the same ends on them, so not all of them fit all coils. Plus with electronic ignition they don't work off 12V and ground anymore, I think some of them work off 12V and -400V for a hotter spark. I would imagine there is a different transformer design to make it work more than 15 minutes, plus probably better insulation for said hotter spark.

There are (at least) two types of coils - resistor and non-resistor. Ours are resistor, because the coil is actually designed to run at 9v. They only bypass this and 'overdrive' the coil to 12v when cranking to ensure a powerful spark.

But you're not supposed to bypass the resistor all the time, otherwise the coil will be overdriven, overheat and maybe burn out. Plus when coils overheat, their resistance goes up and the spark gets weaker.

A traditional or points non-resistor coil won't work very well unless you bypass the resistor, but then you would look the cranking advantage (bear in mind our cars were designed to start in below freezing in Bavaria so probably a non-issue for most).

I've never replaced a coil on any of the old cars I'd had - they generally either work or not.