Rim and tyre choices

It's largely a matter of personal taste. I run 100/90 18 front and 120/90 18 rear on 2.50 akront flange-less rims - on a race bike and it works for me.

On a road bike, I think I'd stay with a 19" front and, to future proof it, choose BMW /7 series sizes, i.e. 3.25x19 front on 2.15 rim and 4.00x18 rear on 2.75 (or 2.50) - there's a good chance that these sizes will be available a long time. They do look "right" on a Commando with the std front mudguard and handling is solid.

Going lower profile on the tyres will reduce ground clearance which might be a problem with std exhaust. Going too wide at the front can cause wobbles and heavy steering. You might just get away with a 130 in the rear but no wider.

Good to see someone thinking out of the box Graham. If you want modern rubber and prepared to fit the recommended rims, Conti Classic Attack radials are available in 100/90 R19 and 120/90 R18.

They use tubes and  2.50" rims both ends rather than the 2.15" front. Equivalent Bridgestones or Metzlers should also work well if your sizes are available

Conti don,t seem to do a 100/90 R18 front radial to give the probably optimum 18" set Stan mentions for his race bike. I used a similar TT100 setup on my Atlas special but with 3.0" rear rim and this was the best of my crossply fitments. Instant improvement. However the 19 front and 18 rear on 2.5" rims would be ideal in your case. 

The grip and mileage of radials on correct rims should far exceed the usual Roadrider/TT100 cross plies and they give a smoother ride. Contrary to popular undying folklore I have found radials work well provided the correct rims are used.

Radials are the way ahead as Neil points out, but the classic attack is a track and road approved tyre unlike many other radials the most of which are road only. The life of these particular road and track approved radials is not great, as many super bike owners have found. You don't get something for nothing. 

Realistically, the classic attack though very good, are expensive compared to Roadrider Mk2, which is more than adequate for the majority of Norton riders. 

You can check your clearance with the tyre widths below or compare to your BMW. Most modern tyre dimensions are the same so your chosen Metzler or Bridgestones  should be similar. Odd man out is Avon whose rear radial tyres are usually wider. 

The 5000 mile life many Roadrider users report is unacceptable in my view for a lowish powered middleweight bike usually ridden in cruising mode (and there,s nothing wrong with that) 

An Access Norton forum member used Roadriders with 2.50" rims front and rear, maybe 19" and raved on and on about them. Unfortunately his loud, confrontational and self opinionated manner alienated too many to take much notice.... Bad bloke to get stuck with in a pub

Conti Classic Attacks

100/90 R19  2.50" rim 101mm wide x 663mm o.d

120/90 R18 2.50"- 2.75" rim 119mm wide x 673mm o.d