I suggest before buying a new clutch centre that you double check the state of the taper on the crankshaft as well. This should be smooth and there should be a number of horizontal grooves (4 I think) running down the taper. If these are no longer present or in any way worn then the crank has suffered as well. You may find you can still get round this with a new clutch centre and by using a smidgeon of bearing fit to hold it in place. But be warned that if it works it will be the devil to get off again probably needing heat to softer the adhesive.
By the way fitting a key into the crankshaft is neither necessary nor desirable. The taper is what provides the grip. A keyway is primarily to provide a positive location for things which need to be lined up correctly e.g. the dynamo stator to ensure timing is correct. Keys don’t provide a secure drive except perhaps in very low stress applications and on an MZ with a worn taper would almost certainly fail quite quickly doing even more damage. Not a good investment for this application.