I also have a 1929 Excelsior, albeit the smallest in the range, not one of the bigger ones. It's been my habit for years to collect as much info as I can on any bike I have so I've accumulated much of what is available on the 1929 Excelsior range.
From your description you appear to have either a Standard Model No7 or a Tourist Trophy Model No11. There is little difference between the two, the obvious one being the Standard has a kick starter, the Tourist Trophy has rear set footrests and no kick starter with no means of fitting one. Electric lighting was an extra on both.
Excelsior advised that if electric lighting was fitted it should be ordered with the machine as it was built in during manufacture. In other words they would fit some sort of mag/dyno arrangement instead of the plain magneto of the base model. Companies such as Excelsior bought electric lighting sets as a complete kit from the relevant maker. This was much cheaper than buying separate items from different makers.
So, whatever make of mag/dyno is fitted also dictates what make of headlight, tail light, etc, is fitted also. This also dictates how the beast was wired up and what wiring diagram you need.
If you have a plain magneto with no dynamo to charge the battery it probably means the bike was bought without lights and an aftermarket lighting system installed. In that case anything goes.
You need to tell us a little more.
Cheers,