Get over it, the smallest spark plug that will work for any length of time is the Rimfire and its clones...1/4-32 thread...
There are much smaller plugs but finding a cap will be well nigh impossible...As well as a way to put it in your engine....
Marvin and Morrison website has the plugs...Good luck...
Look on the conversion forum , find Gas/glow, use glow plugs...No caps at all