Always use new rubbers.Set up the clutch hub on an old main shaft inserted in a vice.Use a large "C" wrench ( a big pipe wrench will work too) to put pressure on the hub to insert the rubbers.If you don't have an old main shaft, it will be very difficult to get those rubbers in.An old main-shaft for any Norton from the mid Thirties up to the Commando will work, so there's a lot to chose from. Good luck!
Charlie