An air tool will often shock nuts on rotating parts loose without turning the component if you struggle to hold it.

The clutch pullers are not complicated and should be available from any Norton parts source. They are used on all post-war pre-Commando models.

RGM certainly lists them.

http://www.rgmnorton.co.uk/buy/clutch-puller-for-amc-gearbox_1047.htm