Peter,
The cam ring may not be seized - if the advance/retard cable is on the long side, it is possible to fit the cam ring with the cable end fitting out of the operating slot.
First, you need to remove the points cover and undo the centre screw retaining the points plate, then with a screwdriver, prize the cam ring out of it's housing. You will then see that the advance/retard cable has an end fitting with a shallow head that locates in a slot on the outside of the cam ring. Operate the advance/retard lever until the fitting at the mag end is in mid position, then refit the cam ring so that the slot picks up the end fitting. Make sure the cam moves as you operate the advance/retard, then refit the points assembly and cover. Finally, re-adjust the cable so that there is minimal free play.
If the cam is seized, remove the points cover and gently warm the end of the mag with a hot air gun - the zinc based end plate on the mag should expand enough to get the cam moving and allow some thin oil (3 in 1) to penetrate between the cam ring and it's housing.
Hope this helps,
Regards,
Eddie
