A "kid" in our club some years ago had a U-Key 35 with a Fox .35 Stunt and bought a short Fox glowplug for it (as recommended by the knuckleheads at his LHS). It would start, but was very touchy on the NV and barely ran. We screwed in a Thunderbolt R/C Long and it was entirely different and relatively civilized, so flying could commence.
Short glowplugs belong in engines .15 CID and smaller. But I guess if you wanted to (and could do the machining), you could lower the glowplug seat enough to get the business end of the short glowplugs flush or slightly protruding into the combustion chamber. That would make them run properly, unless they happen to be Fox or Fireball glowplugs, of course.
Steve