Vespas can be made fast and light. Learn to love their simple design.
Do you have a case bolt rattling around?
What do you mean no nut? Normally the case bolt head is on the stator, and the nut is on the outside of the Vespa motor. I think you might need a true teardown to correct what you've got going on.
On the plus side, if you've got an air leak, there is only one seal, on the fly side, that doesn't require splitting the case to remedy.
Dropping vespa motors is one of my specialties- it's super simple, and gets any work you need to do done three times faster than trying to work on it in the frame.