The older Italian Necchis are notorious for freezing up if they're not thoroughly cleaned and degunked, especially if they've sat for any length of time. The Vintage Necchi Yahoo group has a fabulous tutorial file on cleaning the insides of these machines, I think she used a Necchi Julia for demonstrating the procedure but it can apply to any machine really. I use this method on most of my newly acquired machines now and it makes a world of difference. The smaller screw in your hand wheel clutch isn't broken, one is supposed to be smaller than the other.
The connection between Janome and Necchi is a current one as Janome is now building Necchi machines, there is no Janome connection to your Italian machine.
Cari