Swinging arm.

The gussets (bracing?) were added prior to the MK3 they are on my MK2A however the MK3 has a more stouter X member. There were changes to the swingarm for the rear disc brake on the MK3, how easy they are to replicate on a pre MK3 gusseted swingarm I don't know, plus there is the change to the pin housings for the welch plugs instead of the long 5 BA screw to sort out. 

I would keep your MK3 swingarm and if you have any concerns that the larger X member is not doing its job get two 06.4540 gussets welded on.  Just check the tyre clearance before welding.

https://andover-norton.co.uk/en/shop-details/20774

As John has said, stick with the Mk3 swinging arm (pivoted rear fork to be pedantic :)). Practicalities aside, the Mk3 is more heavily build with larger diameter tubing and a cross member that braces more effectivley than its immediate predecessor. Difficult to imagine how you might improve it without going custom. A solid wheel spindle is worthwhile as is stiffening the engine plates but at the end of the day it's a Commando; the list is endless and the Mk3 swinging arm is way down.

There is a good photo of the mk3 swingarm on the AN site of you want to compare to the one you have, other option is to use the drum rear wheel and a longer brake cable or cross over shaft.

If you're feeling adventurous / not bothered about originality, you can fit a disc brake on to the drum brake swinging arm, as on the Thruxton Racer:

https://andover-norton.co.uk/en/our-bikes/detail/555

It's an 850 swinging arm that has been cut down to remove the drum brake plate stops and a brake torque arm mounting welded on. You could use a Mk3 wheel or, whisper it, a  TZ350 hub. Original Norton Lockheed brake caliper, AP Racing or something else.