In the UK some of the early CXs had Metal breather pipes in the heads and they use a rubber pipe to a T-junction and then this went to the Air-box.Very messy affair.Then later the blanked heads and and extended 17mm Valve timing inspection cover at the rear of the engine like both my Cxs are.I had to blank off the head you see as it was from a spare engine but both my CX have the Extended Timing cover,

As for your right cylinder problem I assume you made the float heights correct for both parts of each float and not just the one in the picture.Both parts of the float have to be even.
Also have you tried re-grinding the Float needle valve using a bit of toothpaste?This is mentioned in my Haynes and I have done so in the past.
Also could still be electrical.Get a pair of snipe/narrow nosed pliers and with the engine running pull the plug cap off the plug slightly.Does the cylinder still fire?