The OPs question was asked in several different threads on multiple forums and answered in several different threads on multiple forums. The reality is the company that made the engines is no longer in business, so you won’t be able to find parts for it soon. Another reality is the best brand of plug to use is often one carried locally as this makes it easier and cheaper to get. Otherwise if one has a favorite, buying online is best.

If your engine is designed to use long reach glow plugs, use long reach plugs. If it’s designed for
mid reach Japanese plugs, use those. Enya is the only one I know of that actually takes the mid reach plugs. OS standard plugs are shorter than long plugs but longer than short plugs. When the plug is proper for the engine, the base of the glow plug will be FLUSH with the top of the combustion chamber. If the plug is recessed, you’re throwing away power and risking some tractability.

Heat range- I always recommend to start with a medium heat plug for bashing. Hot plugs end up being less efficient to use as they often are improperly tuned to and consequently use more fuel. Colder plugs give more power over hot plugs, but the colder you go, the lower the idle quality may be. Not a big deal for a seasoned tuner, but the novice will have better overall luck with a long reach medium heat plug. Novarossi C6S, McCoy MC8, Picco P6S are great standard plugs. For turbo plugs, I’d recommend Picco P6TH or Novarossi CTO6/C6TGC. I prefer their quality over the other brands although O’Donnell has been said to have very good plugs with the same taper as Novarossi plugs. When you pick a brand of glow plug to use in a turbo plug engine, use the same brand all the time. Some heads have slightly different tapers and once they deform the head, they may never seal properly again.

Enough of my ranting. Hope this helps someone out.