Just found a green Necchi BU Nova that is missing a face plate. Any ideas where I could find a replacement?

Try http://www.sewingstyle.com/Necchi-Sewing-Needle-Plates.html - the BU Nova is a 50's era machine so you may have to hunt down a second hand spare part or see if one of the other model plates will fit this model.

Also there are 3 or 4 Necchi Italian machine owner groups on Yahoo, I'd suggest that you join the one called Necchisewingmachineclub and then check out any posts on spare parts and see what you can glean there. Just read the group rules as often they like members to "share" information and don't like newbies to drop in and post a question then disappear. Or ask the same question on several groups (crossposting).

I know that many Super Nova owners love their machines and highly regard them.

Good luck tracking down the part for yours.