be careful with head leaks. If you look on the left side of the engine (while riding) you will see a black cover plate. that cover is directing wind from your flywheel over your cylinder/head cooling fins. You will always feel air there. the thing to look for is if you put your hand real close, you will feel the difference in pressure as the engine goes put put put. the same as the feeling at the end of the exhaust. A head leak might not respond to carb spray though since it only leaks during compression.
I have a hunch that your crank seal is blown. I have had that happen twice. I might suggest a full engine rebuild. $30 gets you a rebuild kit. yo will get all new gaskets and seals and 2 new crank bearings. While you are in there you can bore out the intake. Search the wiki for vespa engine rebuild how to's. It might be easier than you think. A flywheel puller/case splitter is a huge help but not totally necessary, as ong as you have the points style flywheel with the little observation window cut out.
On other quick dumb thing. Make sure your carb is fully inserted onto the intake. and that you have the appropriate rubber o-ring and plastic shim inserted. that is a common leak spot.