Try an "extended tip" glow plug; i.e. an O.S "F"; Fox Miracle; or K&B 4 stroke plug (available from from MECOA).

I have listed the plugs in order of heat range (hottest to coldest). Hangar 9 "H" and Saito SS plugs, similar to the above, may also be suitable, but I haven't any experience with those.

Choose the one that works best in your application.

Many R/C'ers running large 2 stroke glow engines inverted find that putting a 4 stroke plug in (designed for O.S. four strokes) really helps reliability. Apparently the extension on the tip helps ward off oil pooling and snuffing out the coil, which can be a problem at idle ...