Twas not me that suggested the newer type plugs I believe. Your original problem was termination corrosion it sounds like as your thread progressed.

Buy or get your hands on one good plug of the same type to replace the broken one. . Put the system back together and see if it works. If not troubleshoot from there. Getting every connection point really clean is not an option.

The earliest years of the 123 used the same glow plugs and relay I suspect. Glow plugs for sure. Try your Ontario auto wreckers on the web for donars for the relay if eventually needed. Or on ebay perhaps.

Many years since I have been in Aurora. Last time there was to rebuild a single cylinder enfield motorcycle. To give you some indication of how long ago. A rebore of the cylinder with a new piston and rings was 14.00 or 17.00. Thats for labour and parts on the removed cylinder and there was no sales tax back then either I believe.

With the glow plugs out a compresion test on an engine sitting a long time is not a bad ideal. Especially if you can borrow a diesel compression gauge with an adapter that fits the holes. Engines sitting that long can have stuck piston rings. It is always nice to know what you have compression wise early in the game.

I did as you said, and the reading was still 0 ohms.