Looking at your picture of the tool you have there your missing the drill bit that should of come with that tap and the replacement threads. It is a kit usually. You need the drill bit that is the correct size for that tap. You should first drill the spark plug hole with the bit which will leave...
I have started replacing the carburetor instead of rebuilding them. The cost difference is not much, I search the ole interweb and usually find what I need. I think you will be surprised on how little the price difference is.