I have a gallon of DuPont 1080S URO Clear that is about 10 years old. If it is usable, what hardener can I use?

If it is that old I wouldn't use it on something important. It isn't worth it. Start over with fresh paint.

1080s Uro Clear calls for 1082 activator , but most any urethane hardner will work, I have had best results with PPG, But it's very expensive. They say shelf life is about 6 months, but I have used paint that's 20 years old and had no problems with it. If it's liquid, even if it's thick just add thinner. of course anytime it's questionable I always spray a test pannel and make sure it will be ok...... BTW I have a couple gallons of this 1080s uro clear and use it often on parts and small projects but pefer top of the line ( PPG Concept) for customer jobs.