I have given up on testing original installed antique outboard condensors, I just don’t trust them at all. Also, NOS are subject to deterioration over the years sitting on a shelf perhaps near a hot ceiling. It seems that new automotive style condensors or high voltage electronic capacitors are the only good way to go.
I have used 630 volt mylar film electronic capacitors on 4 engines and have been very satisfied, however the body of the capacitor should be bedded in sealant to resist stress on leads due to vibration. See attached photos. The first photo is of a 1949, TD-20, 5 hp Johnson mageto armature plate showing an installed capacitor 
I have found that pure silicon will not hold up to oil and gas fumes, it gets soft and melts. Have switched to Permatex Ultra Grey gasket maker (from Oreilly auto parts). See photo 2. The capicators are NTE brand from Radio Shack, on line.
Bob Zelmer