| not sure M8 drop a email to RD or dazza they may be able to help
if not you can pick them up from daves motors from USA they are well priced & reliable
you may be able to use a fatter plug with the same length reach & have the thread in the dome re tapped to suit a fatter plug ?
or even take it & the plug to a fitter or machine shop & get a quote on them making you one from alloy or steel for strength | 
