A guy Stateside over on the R****R forum just replaced them with a stock coil and plug leads.....just got to match the specs of the stick coil.....think he used something of a Ford pick up.
It's been suggested there that corrosion is the main cause of failure. One of mine was rusty so I carefully cleaned it up and treated it with some Jenolite(?) then plenty of silicone grease/spray on reassembly.
Ohh, and pull the coil off straight, DO NOT rock it side to side at all.....this is where the silicon grease pays off.