You need to get a head shim that is compatible with that motor. The one I bought was Novarossi. The shims come in different widths. I can't rememer what size I am using, but if you do a searh on the forums for 'head shim', the info is posted somewhere.
The shim (which is soft copper) goes between the button and the crankcase around the top of the sleeve. Take off the head and the button should stay fixed to it. Once you have the shim, and you have the head off, it will make sense.