mk3 swinging arm swap or modify

Yes, the drum brake swing arm will fit Mk3 engine plates.

Modifying the Mk3 swing arm to accept the drum brake will entail more than 'just' welding on a brake stop (which would be better fashioned from a piece of new steel rather than ruining a perfectly good pre-Mk3 swing arm).

I have all the parts you would need convert a Mk3 to drum brake (from a Mk2A), maybe we could work out some kind of swap?

Unfortunately Martin, I'm not convinced that the later factory drum cush design is very effective when compared to the MK3 setup, as the pads are so small, so effectively you will end up having minimal cushioning in your power train?  Got to be slightly better than none at all mind.

I reckon a better engineered alternative is the Madass version, designed to fit, but that is $$$'s of course:

https://tritonmotorcycleparts.com/297620484/297620488

You might be able to adapt a rear wheel cush drive hub from a different marque?  I've heard of the BMW 650 single cylinder hub being adapted, plus with guaranteed spares availability, but you'll no doubt need machining facilities.

My ideal would be a cush drive front engine sprocket, (like the one Norman White produced prior to his retirement), together with Don's rear drum hub.   IMHO the Commando transmission needs as much help as you can provide it with unless you are a gentle rider or have completed the fantasy and fitted a 5-speed TTI gearbox. 

Me and my MK3 - I'm also considering swapping the pedals over, bit I'm sticking with the disc rear wheel, and don't have the brake seizing issues that you mentioned.

I'm intrigued that you have been running around with your hydraulic rear brake pedal swapped over to the left side - could you indulge me/us with a photo please?

What is wrong with the 'seizing' disc brake? Does the caliper have original chromed steel pistons (that are prone to rust). Replaced with stainless steel ones cures this.
I still have the brake pedal that came off a Mk3 that came to me for work doing. It had at some stage in the past been converted to right foot gear change and the brake pedal pulled a cable that looped around the bike to operate the rear master cylinder (still mounted on the right foot rest).
This was removed and the bike put back to std (left hand change) as the owner had other modern bikes with left hand change. Going to a drum rear is a backwards step (especially with the earlier set up, miniscule rubber buffers = hardly any cushioning effect, etc) 

Roger Myers has some thoughts on the long/short subject here: https://www.rgmnorton.co.uk/buy/swinging-arm-spindle-long-standard-size_2732.htm

You can fit either length and the RGM pins come with both cotter slots and centre bolt tapping.

I'm was horrified when I learnt that the factory didn't produce a properly engineered pivot retaining set-up until the Mk3.  Six years plus is disgraceful.

I don't have any experience of the earlier long-pin set up, but assume it was shortened to make room for the new lubricant retaining felts?  And were these introduced because the original seals tended to leak oil, and maybe lubrication then often ignored by Owners?  Dunno.

It initially seems clear that longer pin and bushes would be advantageous to fit to a MK3, but how to improve lubrication and oil tightness?  What combination of seals, felts, and O rings would be best?  Do the felts have to be discarded, and is it wise to do so?

Norman White covers the refitting of a long spindle to an early long spindle model:

"Rather than assemble the original O ring and shield, upgrade to 850 MK3 seals Part No 065227, which will fit without any alterations", and  "remember that new bushes often require reaming, contrary to expectation".    Page 44.

Which end cap arrangement is also best, assuming there is room for either, I have no idea.

As the earlier inadequate 1/4" pivot securing screw is now redundant, wouldn't that be a good place above the pin through which to top-up the oil?  Even fit a small oil holding reservoir?