chook,
Before you go buying any parts I would be doing the following:
1.Clean the points with emery paper
2.Clean the flywheel magnets with emergy
3.Clean the earth contacts (where the 'E' plate sits on the engine)
Then check what the spark is like with a new plug.
The cost of points & condensor would be about $14 max and the coil would be about $30+.