Steve,

The one I have does also have a wire from the magneto housing to the spark plug but it also has two coming from the bottom of the magneto/coil to a set of points and a condenser. The wire from the points, condenser and magneto/coil are all soldered together with the other wire from the underside of the magneto/coil going to earth. The spark plug HT wire come from the top of this magneto/coil part..

Hope that makes sense.

Cheers

Cad

PS I got it running yesterday, but it's running very erratic, and only with the choke fully on. Guess I'll have to take the carb apart and clean all that up.

Cheers

Cad