Replacing wiring

...should be along shortly! 

... are reportedly the Mk3 ones. I have a Mk4 on my Electra which is supposed to work down to 7 volts but I'm not 100% convinced that it does. I've checked the voltage while cranking and it's just over 10 volts which should be plenty. I think I'll try strobing it while cranking to see if that shows me anything.

Grant Tiller has produced some nice wiring diagrams and is a member here.

To go Negative Earth on a MKIII you almost have to go to a regulator/rectifier (called by Boyer Brandesdon as a Power Box). Unfortunatly these Power boxes are over priced compared to what else is on the market. But the Power Box does include the Capacitor (2MC) which is recommended, and is available else where.
The ignition coil is easy enough, there is two spark/HT lead outputs, they just feed the two spark plugs. Then there is what appears to be two Luca terminals (spades) these go to the ignition unit and either 12V or earth depending on your electronic ignition system. The two Luca terminals will be very low resistance to each other, while the two HT terminal will have a very high resistance to each other (perhaps several K ohms). There will Not be any connection from the HT connections to the Luca terminals, nor will there be any electrical connection to the metal mounting holes.

Peter,

There is a huge selection of wiring diagrams on my website which cover the MK3 and the majority of aftermarket electronic ignitions and reg/recs.

While there are a few diagrams on my site wired for negative earth, I would question why you want to do this?
 

In my opinion, there are only a couple of reasons that you would want to do this - for example:

1. Fitting something like a Motogadget wiring unit, which is negative earth only.
2. You have a garage full of 80s jap bikes, which are all negative earth, and your Norton is the only one running positive earth - so you feel you are a danger to yourself when you work on it.

For me (personally) unless you have that good reason there is just no point in doing it for the sake of it.

It is a nightmare for people like me to help you - usually I can look at a pic or read a description and talk someone through fixing their issue. If you deviate too far from factory standard, that ease of diagnosis is lost.
Most classic bike electrical parts are designed around positive earth, and the majority of the wiring diagrams for these parts are for positive earth.

Most roadside help (be that a mate helping you out when you break down or a recovery professional) will look at a 70s Norton and assume it is positive earth. Particularly if all your red wires (and there are a lot on a commando) are earth.

Using an existing harness rather than starting from scratch, most people that swap from positive earth to negative earth tend not to bother building a new wiring harness with the correct colour cables.

Of course, it’s totally your choice - this is all just my opinion.

My ES2 and Jack Moore Twin both came with Negative Earths. I believe that immediately post WW2 this was not uncommon. Many books mention that using the chassis for Negative Earth reduces corrosion issues. Surely the answer is to run separate supply and return electrical cables and just use the frame for hanging the engine and wheels from???

for the ignition system. Although running dedicated earth wires is also a good idea in my opinion. As to negative / positive earth, I do find it easier if all my vehicles are the same but do stick to original setup. My 1952 ES2 was negative earth; the Electra positive.

The solution is simple - 

Tri Spark ignition - no black box to hide away, on a MK3 despite it's size is not easy to find a spot.
Tri Spark MOSFET rectifier regulator - it provides 14.3V at tickover if all the alternator output is fed to it.
Al Oz battery status monitor - does away with the assimilator and indicates very well what the DC is doing, not like the old system that just makes a light go out when the alternator spins fast enough. 

Stay positive earth, yes, there many arguments but my bike is 50 years old and it has not suffered from corrosion for being negative earth. 

The MK3 has a nice thick earth wire to the engine - MAKE SURE THIS IS CONNECTED TO THE ENGINE IF THE BATTERY IS ON THE BIKE. Twice in 30 years I have been working on the ignition and stood up and caught the start button, if you do that without the thick engine earth wire connected to the engine, guess what the, the large amount of amps will try and go via the thin earth wire connected to the head if fitted back to the battery, unsurprisingly the loom melts real quick. 

As Grant says, the Tri Spark on the MK3 is a doddle to set up, use the timing slot on the crank that is visible when the blanking bolt is removed form the RHS crankcase. Rotate the crank and set the slot in the middle of the threaded hole, mark your degree scale so you know that is the 30 BTDC irrespective of what the scale reads. Set up the Tri Spark, refit the blanking bolt and the bike should start first kick.