ssxl's are fun. I'm assuming the other Garelli's you've got lying around are Gulp style with the verticle engine. That nut is an M9 in 1.0 pitch, as in, non-exsistent. Only place I've gotten them is blown up engines and Mopedjunkyard. if the threads on the crank are fucked, you might try recutting them to an M8 which you can get anywhere.
There is no top bar, it's just like it appears. You should check the bracket that supports the tank and seat, the welds that hold it to the frame tube often fail, which can lead to your seat and tank collapsing inward.
as for spark, the simplest running wiring for an NOI is black to HT, blue to ground, yellow doesn't matter.
If you need parts, email me. I've got shit I need to get rid of. Also, verticle engine parts are not interchangeable with horizontal, at least not in any useful way.