angle of the spark plug to the bore line of cylinder?

The spark plug is 28.8 degrees from being in line with the cylinder..

You will only have room for a very small rod through the plug hole when in line with the cylinder.

A piston stop will give you a better result even with the cylinder offset. Jim

Just realize almost 29' puts way more sideways bending force on probe than force along its shaft so be gentle. A solid alu threaded shaft screwed into chamber might be best way for Norton to stop against.