Has any one rethreaded sparkplug holes

 for a larger spark plug

  What I have is a set of screamin eagle heads , Evo that take a 12mm sparkplug. i am running a magneto for inection and cannot find the right heat range. I would like to  bore out counter bore  and tap to run a accell 137. whitch I run in my 62 ch whick is a  1 or 2 kick bike. Oh did I say this is a kick only bike.  or am I going to have to find a set of stock heads. and machine those. 

  any  ideas?