If you Google Casa Lambretta, I would say most of the shops on the first page should be able to help you out. I've personally had parts from most of them & if you stick to the larger dealers I'm sure you'll get sorted. In fact if you buy the manual everybody's recommending there's a list of reputable dealers in there as well..
Good hunting....Mark.
Btw I'm surprised you're not getting more posts from people who are in the process, or have rebuilt Luna line models... perhaps you should start a new thread specifically about your engine build so you can gather as much info as possible..