Hmm, dunno. All three bikes were geared low when I got them. It's a Minarelli thing, I guess.
Oh, and Tomtom, most Minarelli v1's are equipped with a 9, 10, 11, 12, or very rarely, a 13 tooth front sprocket. I think the 16/18 dealio you are thinking of is for Puch. Once you know what yer front is, then you can calculate what you would need for your rear.