What Alternator Stator ?

Can't advise on new alternators. 

These are good regulator/rectifier boxes.

https://www.ebay.co.uk/itm/317112493018?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338778191&toolid=10001&customid=130832X1595795Xd14bf1860384640515ca137e80f32af5&subId2=15

Plug and wires.

https://www.ebay.co.uk/itm/317112536052?mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338778191&toolid=10001&customid=130832X1595795X7f503c9c8f3b448b6ff978debd99bab4&subId2=15

Just ask Al. 
Oz that is... 

The original stator would have had a 70mm rotor (and stator to match) but this size rotor/stator or no longer available as new. So you are stuck with the later 74mm rotor stator, the basic item being the RM21. LED lights will reduce any demand for electricity, while indicators are intermittent usage (maybe even LED) so hence are not to worry about, With regard to rotors-these are magnetically /electrically all the same, but with age the rotor might be coming apart and its magnetism can reduce, can be tested (see my web site). Only other area of concern with regard to running indicators 'on all our bikes' is to make sure we have a decent battery, none of the 'gel' ones from the alarm world, but something with reasonable AHR ie you can get 7Ahr and even 9Ahr in a Dommie battery box.
With regard the reg/rect recommended by M Ball, we do NOT know if this is for permeant magnet rotors, and it is for 3 phase this is not for use on an RM21 (single phase) My advice is stay clear of this item unless you KNOW what it is for and how it is meant to be used. ie on specific Hondas

The generic Chinese "CBR600" reg/rec I linked to is in use by me on a permanent magnet, single phase alternator.  I've fitted and used a few. Many other old bike riders use them. They perform well and last well.  They show precise voltage control and don't get hot.

That's why I recommended it. I'm surprised Al Osborn doesn't know about them.

As my alternator is single phase, one of the reg/rec's yellow wires is not connected. Some people connect two yellow wires together to one of the alternator output wires.

It might even be worth an expert's time running a few tests on one.  After all, it's not a big investment at £11.53.

The early Indian Enfield Bullet stator would appear to be dimensionally correct. Try Hitchcock's for advice and supply.

If the tatty old stator is the type with exposed coils, it's a fair bet it would fail at some point.

Don't throw it away!

This is the colloquial name to the very 'square' cornered batteries that have 1/4 Luca terminals, they are made for the burglar and fire alarm world. They are meant to be float charged for a long time and have a lowish  limited charge rate (depends on the Ahr as to how much) If they ever get over charged for any length of time they will dry out and fail. They are not meant for Automotive use, which is highish short term charge/discharge. Our bikes were made for the Flooded acid type which when they overcharged (the earlier 6V) would tend to boil off, (you then topped them up). The gel type if they over charge dry out and fail-end of. AGM seem to be very good for our bikes.

I wouldn't go for less than a 6Ah battery.  More capacity, the better. Traditional lead-acid or gel. I dropped one bike twice and never had a problem with acid spill on the old Lucas batteries!  Perhaps I led a charmed life when I was younger! :-) 
Yuasa are an excellent make and their 6N11A-3A is 11 Ah (!!) and only measures 122mm long x 131mm high x 62mm deep. Their website lists them at £39.55.
  
Of course, you can get a Powerline 6Ah, AGM battery (6N6-3B) for £13.47 from eBay.  I have no idea how good they are, but I would prefer a capacity closer to the old Lucas batteries in the 1950s and 60s. The PUW7E in my 1952 Lucas catalogue lists them at 12Ah at the 10 hour rate! I've copied the relevant page here : -
 

Just recharged the Yuasa acid battery that has been neglected for a few months on my out of use 250 Ducati that i used for the IOM for many years. The Voltage slowly raised to 14.4 .  I then noticed the date on the side  2001.   thats right 24 years old.

Fact eBay does not sell anything-eBay is an auction/buy it now site, for merchants, you do NOT buy from ebay you buy from the appropriate merchant.
Regarding batteries on our bikes- yes 5Ahr is my consideration as the SMALLEST you can fit. Anything smaller will over charge on a decent journey 'over 50 miles' if you have 9 or 11Ahr you will have reserve, when you use the indicators to turn with engine at tickover, When you leave the lights on, when you stop at the lights with the brake on-AHR will help.
Don't expect the 'wonderful' Automate type battery charger to charge an older tired battery-use an 'old fashioned' battery charger and leave it on for a good few hours at a few amps. The 'stupid' automate type monitor's a high voltage and the LED goes Green, but there is no AHR behind it.

I'm always happier with a bigger battery!

If using one of Al's magic regulator boxes, do I just connect the alternator WG and GB wires to the V Reg 2 inputs? What should I do with the stator GY wire? Does it just dangle in the wind (figuratively) or does it need to be connected in with one of the other wires (presumably the GB).
Over to you Al! 

Don't tell me, you have lost the instructions......All A regs come with proper instructions. If it was a two wire stator you would have to use the WHR/GRN and the GRN/YLW, so they are the first choice.
The GRN/YLW being the High Output with respect to the WHT/GRN. The GRN/BLK being the Low output joins with the GRN/YLW for full out put-we all know this!

I can't be responsible for your filling system, you get proper instructions but you loose them!
You can lead a horse to water but you can't make it drink. Fools 10 a penny.
Good to know you are still there Gordon, perhaps the instruction above is helpful to others!!