Another possibility is to remove the separated ring and get the proper parts assembled prior to the job.It is a normal bolt. Counterclockwise to remove. Be aware that the bolt is one time use and that replacement bolts need to be torqued to a High torque value followed by a degree factor which varies depending on which type of key and bolt is used on your vehicle.
In a pinch it might be possible to reuse the bolt a second time but I would replace it promptly upon getting to a location where this was possible.
Replace the seal for sure. It's a $15 part and there's no reason to risk a crank leak for 15 bucks. You will need to make a tool to push the seal in. You can use the old crank bolt and a couple of very large washers with a piece of wood as a spacer to make a cheap and quick DIY tool.Yep, definitely getting the washer as well. I wonder if I should get a crank case seal just in case. I've been told there usually isn't an issue damaging it when removing the pulley though.