[rotorwrench]

An induction coil for ignition has a high step up to it so it has a higher back flow of voltage that a vibrator coil. It likely won't have near as much back flow of voltage. All they had back then was wax paper and foil to make capacitors with so it may work just fine using a modern cap like that. Most horns don't have a cap at all. The 6-volt system is why many older vehicles had caps. Folks don't really use the horn enough to ever wear one out. In the case of these old antiques, it is mostly time and the elements that do them in.