I suggest that you being by acquiring schematics for the machines, assuming you don't have them.
You can probably download them from the pinball database IPDB.com
It would be better to have paper copies so you can highlight areas that you test that work properly, in the effort to location problem areas.
As far as missing parts, I am not sure exactly where to look.
You might start here: http://www.pbresource.com/index.html
Good luck on that.