Fresh gas wouldn't hurt but if it has a spark then don't bother with the new plug and like I said you can mount it to anything drill a hole and stick it on the sub frame. Mounting and grounding are not the same thing, if it wasn't grounded correctly then there wouldn't be a spark. that looks like the CDI unit and as far as i know this is how it should be hooked up LINK as long as no actual wires are disconnected from anything you are fine.
I had some old gas once and ran it in both my moby 50v and my kinetic and the moby eventually didn't run, but the kinetic didn't seem to have a problem with it, but each moped is its own animal I suppose.
After reading what your posts again, do you think the guy used the right gaskets on the cyl and head? Wouldn't hurt to take off the cyl and head to check it out, you can also clean any carbon build up that's on the piston or in the exhaust port while you're at it (edited)