Get the Wellman 070 plugs. They are self-limiting, meaning they will not blister if overheated like the older ac13g's. It is what the military uses for replacements. The small piece of broken tip has probably been blown out of the exhaust. Do you have a small magnatized screwdriver you could put into the glowplug hole? It could also still be in the gp chamber. I wouldn't worry too much about it though.
It's up to you, but the starting sytem always works best when left stock. Instead of spending time and money converting it to 12v, put the effort into getting it to work the way it should. They will start without help in the coldest of weather, but you also need the glowplugs for starting in warm weather when the engine is cold. When left stock, you can still use the manuals for diagnosing and servicing. Gets tougher when things start to get modified. They work great the way they were designed.
When I first got the truck the system did nothing at all. I wiggled some wires attached to the module under the dash(the little black box) and that is when it started working somewhat. Not sure if the black box needs to be changed or not. Also if I do convert it to a 12V system one day will those wellman 070 plugs still work.
Sounds like your problem is in the box or card. Pull the card and clean the contacts on the card and the box. Make sure the card is pushed in ALL the way when you put it back in the box.
Since we never know the maintanance history of these vehicles, for insurance and a good baseline I would also replace the gp relay on the firewall. They are known to fail over time. It is a good idea to replace the starter relay under the dash too. Early originals had a design flaw that caused them to stick 'on'.
One last thing, get your fuel system up to par by replacing the fuel filter and mechanical lift pump. The pump gets weak over time too.
Also, the engine needs to be turning a minimum rpm when starting. If it's not turning fast enough it will start hard or not start at all. The starter slows down over time and causes hard/no starts and could also cause excessive wear on the flywheel. A good starter/alternator shop can rebuild them for around $125. Cheaper than a new 12v. Don't forget to always reinstall the starter brace too.
Hope this helps.
