High Plains Thumper wrote in
:

Well, if you think about it, if you have access to a machine
shop with a MIG/TIG welder and milling machine or someone who
is competent and does, yes, could be a solution. However if
it is finding a small engine spark plug that will do the exact
same thing in a larger diameter, I'd think that would be the
easier way out.

AFAIK, 18mm spark plugs are not particularly scarce, any other solution
seems to me to be more trouble than it's worth.

Ace Hardware carries them for small engines:

http://www.acehardware.com/sm-small-...fi-1305598_cp-
1254884.1255120.html