Go to a wrecker and get used ones. The cables won't be in great shape after 40 years, of course. Some bikes used coils that will fit and have replaceable cables - check impedance thjough, many are low for factory electronic ignition. You want 5 ohm minimum primary coil resistance.
Dyna coils are a good product. You have to file a very small bit off the stock support towers, afterwards they will still work fine with stock coils. If that's too painful you can make up new towers fairly easily and keep the OEM ones in a display case.
