Disconnect the dynamo wires.
After removing the timing cover and cleaning out the old grease undo the dynamo strap and rotate the dynamo to loosen its chain.
There is no need to remove the sprocket on the dynamo.
If you are going to run the engine without the dynamo, remove the dynamo chain. To remove the chain you have to remove the large dynamo drive sprocket. I used a mixture of leverage and light taps to free the sprocket from its taper. You can now remove the sprocket and chain.
I was not able to wiggle my dynamo through the engine plates, (I don't know if it's supposed to be possible) so I removed the drive side engine plate. Be carefull to look after the large cork washer that seals the dynamo to the timing cover unless you are going to replace it.
I made a blanking assembly by cutting a disk of ally sheet to cover the timing chest dynamo mating surface and held it in place by screwing it to a piece of wood inside the case.
When reassembling rotate the dynamo body to set the chain tension; while tightening the dynamo strap make sure to press the dynamo into the timing case to seal it against the cork washer. Check the dynamo chain so it has no more than about 1/4" slack. You'll need to rotate the engine and check the chain in various positions to ensure the chain is not too tight or too loose.
Put a large glob of soft grease in the dynamo chain-case. It might be useful to lay the bike over while doing this so grease doesn't run onto the timing cover mating surfaces. I use Hylomar Blue for sealing gaskets.
Engine plate on, reconnect dynamo, jobs a goodun!
I'm expecting someone with a lot more experience than me to tell me of my errors in the above!