I’ve struggled to find key blanks for 70’s bikes in the past, but found a guy on eBay that sells them - hopefully this is a link to the right key for Guzzis:
http://www.ebay.co.uk/itm/2x-Fiat-Alfa-Detomaso-Ferrari-Iveco-Moto-Guzzi-X1-9-Key-/301988202061?hash=item464fe6464d
I bought a couple and took them into Timpson’s today to get them cut and she told me they actually have the right blanks as “HD SF5” which is a Ferrari and Fiat key. I’ve made a note for next time.