All of the older British motorcycles and cars that I have come across are Positive Earth. This is the reverse to North American vehicles and is done in order to allow the electrons to flow the proper way such that driving on the wrong (left hand) side of the road is possible.
Seriously, it's no big deal provided you take note of the polarity and make yourself think in reverse with all things electrical on the bike.
I switched one of my Vincents from it's original positive earth setup to negative earth so that I could use a new negative earth only charging system. It was no big deal, I just hooked the battery up the other way round.
Can't comment on switching the Norton, mine is still positive earth and there may be some aspects of the standard electrical (zener diodes, starting capacitor?) which need to remain positive earth. Unless you have some major piece of electrical equipment to install and this equipment is negative earth only, there is no need to change to negative earth.