Instead of a thinner gasket, why not just machine the top of the cylinder to drop the head down ? Certainly easier than "hand jobbing" a thin gasket.
As for the spark plug, a retracted tip type plug does not protrude below the flat surface of the head like a projected tip model does.

The kart was made in the UK by ZIPKARTS who are alive and well and still very much in the business --- Hoddesdon, Herts, England if memory serves me correctly. I saw the owner, Martin Hines at the Monaco Vintage event this time last year.

After reading about the Clark engine, I remembered that I have a strange example in my colection that I have no idea about--its called a SAWYER---anyone ??