The bad news is that the armature has failed big time. The good news is that it is fixable, can be upgraded and the only stumbling block is the money.
Once the dynamo is off the bike, the type can be identified by the model number stamped on the body. Early A's used a so called short body, E3H version, later bikes have a higher output E3L, with the longer body, both rated at 6V output. Reckon it will be an E3L, same basic version used over all A series Plunger and S/A Bikes. Yours looks to have had some abuse already...look at that screw in the centre of the ball race. Once off the bike, two long through bolts hold the whole thing together. The armature complete with its bearings, drive cog and bearing carrier will then slide/tap out out of the body when these bolts are removed. Alternatively remove that slot head screw and remove the armature from the brush end bearing. Extracting the drive cog then presents no problem as the armature can be held still. Cog can be very tight on the shaft taper.
You could just replace the dynamo, like for like, or swap the armature, used or new repro. As the control box is suspect, this must be checked, as it is probably the cause of the failure, again it may not be fixable. But if you can put practicality above originality, an opportunity to upgrade to a new full 12 volt system and solve the problem. The existing dynamo housing can be rebuilt with 12Volt innards.
Plenty on the Forum about the merits of electronic control boxes and the benefits of moving to a 12 Volt System. Plenty of options, all is not lost.
Swarfy
Additional. Worth posting the pictures of the damage here...
https://www.a7a10.net/forum/index.php?topic=15504.msg136896#msg136896
