Brent makes a good point about having the best restored and maintained car you can.
I have found that an electrical test meter is very handy to trace minor electrical problems on the side of the road. I like to carry electrical tape, a small roll of 12 ga wire, a set of spark plugs, points, condensor, rotor button, headlight and tailight bulb, and a carb gasket. That and a little knowledge will cover about 95% of roadside repairs.
I know a man that has an extra carb, generator, starter, coil, distributor, plus enough small parts to rebuild most Model A's under his seat at all times.

As far as I know he has never needed any of them.