Horn question for AC 1974 Vespa Rally 200

Hello, I've been looking everywhere for a 6V AC Horn that will fit the horncast (85mm) on my 1974 Rally 200.  When I purchased the scoot - the horn was non-functioning, which is understandable, considering it was a 6v CC/DC horn.

On the SIP store, I've found two that *might* work - but it is unclear if it will work with the horn:

http://www.sip-scootershop.com/EN/Products/70302000/Horn+V5090PVET3GLRally.aspx

and

http://www.sip-scootershop.com/EN/Products/70301000/Horn+V5090PVET3GLRally.aspx

 

They both, in the item description, say that they fit the Rally 200, but one of them says that the diameter is 70mm.  I know that many of the non-USA Rally 200's were shipped from the factory as AC, with no turn signals... so I believe there MUST be a valid AC horn option out there somewhere!

 

Any help is GREATLY appreciated!

-- Bennett