the other thing i've started doing on vespa builds is putting nuts and washers right up against the head, then the decomp stuff i attach with a second nut. you have to bend the decomp arm a little because the geometry changes, but its a much better way to put the head on.
i used to notice on the re-torque, they would loosen up a lot with the decomp parts all jammed up under there. when you re-torque it with the double nutted setup, you gotta take the two nuts back off, but they don't loosen up near as bad.
head and base gasket leaks are probably the biggest bugaboo on vespas, you're on the right track checking it before you put the engine back in the bike and sorting it out.