I just replaced the ignition switch in the ZD21. Real easy to do, probably took all of five minutes to remove the old and put in the new one. The hard part was removing the shielding panels then reinstalling them. Laying on a creeper and operating by feel and a mirror. That took about 30 minutes.
The switch has an outer cover which unscrews, left to loosen. I used a set of channel locks to remove the cover, don't worry about marring it, you will get a new one with the new switch. Remove the old switch from the hole, unplug from the wiring harness, plug in the new switch. Insert the new switch in the hole, there is a groove in the hole and a nub on the switch so you can't install it wrong, then screw on the new cover. I used a rag to protect the new cover when I tightened it down slightly with the channel locks.
The new switch from Kubota cost $54.18 before shipping and came with two keys. You might be able to get the broken key out of the old one, after removing the switch and turning it upside down and shaking it or give it a shot of compressed air to see if this will dislodge the broken piece. If this does not work refer to the above.